Counting the ones between, not the ones at the ends
The arithmetic on an interval is the bit that goes wrong. A 68 foot run with a 100 foot interval needs no cleanout in the middle of it — the interval is a maximum distance between accesses, and 68 is inside it. Divide 68 by 100 and round up and you get 1, which is one too many, because that count is of the gaps rather than the accesses between them.
The page counts interval cleanouts as the number of gaps minus one, then adds the ends and the direction changes separately, because those are counted on different grounds. A 240 foot run at the same interval needs two in the middle: three gaps of 80 feet, two accesses between them, plus whatever the ends and the bends need.
Direction changes are a code question with a threshold
Every code has a rule about changes of direction, and it is usually expressed as an aggregate angle rather than a count of fittings. Two 45 degree bends may add up to the threshold where one would not. Long sweep fittings are sometimes treated differently from short ones. Whether the rule counts the run as a whole or each change individually differs as well.
That is why the form asks how many direction changes need a cleanout rather than how many bends there are. Working out which of your bends count is a reading job on your own adopted code, and it is not one a calculator can do without knowing which document you are being inspected against.
Support spacing is a material question
The spacing comes off the pipe manufacturer data sheet for the material and the size, and it is not the same for everything. Plastic DWV wants carrying more often than cast iron does, and both want more support horizontally than vertically. A run supported at the spacing for the wrong material sags between hangers, and a sag in a drain is a belly that holds water and catches everything that goes past it for the rest of the life of the building.
The count on this page is one hanger every spacing, plus one at the far end. The line worth reading is the last gap, because when a run does not divide evenly the leftover piece at the end is where a support gets left out.
What the takeoff does not include
Pipe, couplings, cleanouts and hangers. Not the wyes and bends that make the branches, not the transitions between materials, not the sleeves where the run passes through anything, not the solvent cement or the gaskets, and not the fittings the fixtures themselves need. It is a skeleton for the order rather than the order.
Questions people ask
How far apart do drain cleanouts have to be?
That is set by the code your jurisdiction has adopted, and it varies with the size of the line. It is also amended locally often enough that the figure in circulation for one place is wrong in another. This page asks you for the interval rather than holding one, because a page that carried a number would be quietly deciding which code you are being inspected against.
Why does a 68 foot run at a 100 foot interval need no cleanout in the middle?
Because the interval is a maximum distance between accesses, and 68 feet is less than 100. The mistake is dividing the length by the interval and rounding up, which counts the gaps rather than the accesses between them. Interval cleanouts are the number of gaps minus one. What the ends need is a separate question with a separate rule behind it.
How many hangers does a drain run need?
One at every spacing plus one at the far end, at whatever spacing the pipe manufacturer gives for that material and size. The number is easy; the ones that get missed are at interruptions the division does not know about — either side of a beam, at a change of material, and in the short leftover gap at the end of a run that did not divide evenly.
Does the count include the fittings for the branches?
No. This counts pipe, the couplings between stock lengths, cleanouts and supports. Wyes, bends, transitions, sleeves, cement and gaskets are all separate, and so is everything the fixtures themselves need. Treat it as the skeleton of a material order rather than the order itself.
What makes a cleanout useless?
Not being able to get to it. A cleanout behind finished drywall, buried under a deck that was built later, or facing into a space too shallow for the machine that would use it has cost money and bought nothing. The count is the easy part of this job. Deciding where each one physically lands, while the ceiling is still open, is the part that pays off years later.