Three measurements, and why the smallest wins
Take a tape to a shower opening at the top, at mid height and at the curb, and three different numbers come back. In a tiled alcove a quarter inch of spread over six feet is unremarkable; half an inch happens; more than that happens in older houses and in work where the two walls were built at different times. None of that is a defect anybody is going to fix for you.
The glass has to fit at every height, so it comes off the narrowest of the three. Everything above that measurement becomes a gap somewhere, and the gap has to be taken up by hardware. A wall profile or U-channel has a published adjustment range, a wall-mounted hinge has its own, and the question on any job is whether the spread you measured falls inside what your hardware states. This page reports the spread and the per-foot figure. It does not tell you whether that is acceptable, because the acceptable range belongs to the hardware maker and varies enormously between products.
The allowances are hardware figures, not rules of thumb
Two fields do most of the work: the allowance at each wall and the gap between door and panel. Both are net figures, meaning what the hardware takes off the glass after accounting for how far the glass sits into a channel. A U-channel that is three quarters of an inch deep and takes the glass half an inch into it does not remove three quarters from the panel width.
Every hardware maker publishes these. They differ between a wall profile and a hinge, between a glass-to-glass hinge and a wall mount, and between one manufacturer and the next. Getting them from the supplier of the hardware you have actually bought is the difference between a panel that goes in and a panel that is scrap, because there is no adjusting a piece of tempered glass after it is made.
Why the door width matters twice
The door and the inline panel share whatever the opening leaves after the allowances, so setting one sets the other. That is the obvious part. The less obvious part is what the door width does to the hardware.
A door weighs what its area and thickness say it weighs, and that load hangs on the hinges. But the load also acts at roughly half the door width out from the hinge line, so a wider door of identical weight puts a larger moment on the same hinges. The page reports both the pounds per hinge and the moment in pound-feet, because hinge makers rate their products by maximum door width as well as by maximum weight, and a door can be comfortably inside the weight rating and outside the width rating at the same time. Both limits are the manufacturer figures and neither is published here.
The height, and the two gaps in it
Glass height is the opening height less the gap under the glass and less however far the glass stops below the top. The gap under is usually a deliberate clearance to the curb or base, with a sweep or seal fitted afterwards, and its size is a decision about drainage and draught rather than about glass. The gap at the top is often zero and is sometimes forced by a header, a soffit or a sloped ceiling.
What the page does not do is decide either figure for you. Both are inputs, both come from the enclosure design and the hardware, and both change the glass order.
The part that has no adjustment
Tempered glass cannot be cut, drilled or notched after tempering. Every hole for a hinge, every notch for a handle and every finished dimension has to be settled before the pane goes in the oven, and a mistake at that point is a new pane rather than a corrected one. That is the reason a take-off like this one gets done carefully and then checked by the glazier who is making the glass.
Where safety glazing is required, and what qualifies, comes from the code your jurisdiction has adopted and from the inspector. This page does not state either. What it will say plainly is that a shower is a wet floor with a hard surface and a lot of glass in it, that a pane fails without warning when it fails, and that the specification for what goes in an enclosure belongs to your glazier rather than to a calculator.
Questions people ask
How much out of plumb can a shower opening be?
Not a figure this page publishes, because it is not a property of showers — it is a property of your hardware. Wall profiles and hinges each state an adjustment range, and those ranges differ a great deal between products and between a channel and a hinge. What the page does is measure: it reports the spread between your widest and narrowest measurements and the equivalent per foot of height, so you can hold those numbers against the range your supplier states.
Why does the glass come off the narrowest measurement?
Because the panel has to pass every height, not just one. If the opening is sixty inches at the curb and sixty and a quarter at the top, a panel cut for the top will not go past the curb. Cutting to the narrowest and letting the hardware take up the rest is the only order that works, which is why the page uses the smallest of the three measurements and shows you the other two beside it.
How wide should the shower door be relative to the fixed panel?
The page lets you set the door and gives the panel the remainder, because the split is a usability decision rather than an arithmetic one. What is worth knowing before you choose is that door width has a second effect: the door weight acts about half a door width out from the hinge line, so a wider door loads its hinges harder even at identical weight. Hinge makers publish a maximum door width alongside a maximum weight for exactly that reason.
How much does a frameless shower door weigh?
Its area times the weight per square foot of the glass. Three eighths inch glass is about 4.88 pounds a square foot, so a 24 inch door at the 75-5/8 inch glass height this page works out is 12.6 square feet and about 61 pounds. The page works this out for each piece from the sizes it calculated, and it splits the door weight across whatever hinge count you enter. Whether those hinges carry it is a rating from their manufacturer.
Can the glass be trimmed on site if it does not fit?
No. Tempered glass cannot be cut, drilled or notched after tempering, and any attempt destroys the pane. Every dimension, every hinge hole and every notch is settled before it goes in the oven. That is why the allowances on this page are hardware figures from your actual supplier rather than typical values, and why the finished take-off is worth checking with the glazier who is making the glass before anything is ordered.