The two numbers that decide everything
Crown is a flat board held at an angle across a corner. Everything else about it follows from that. The face width is the distance across the flat back of the profile, and the spring angle is how far that back leans off the wall. Multiply the face width by the cosine of the spring angle and you have how far the moulding drops down the wall; multiply by the sine and you have how far it reaches onto the ceiling.
With the defaults in the form — a 4.625 in face at 38 degrees — that is 3-5/8 in down the wall and 2-7/8 in onto the ceiling. Change nothing but the spring angle to 45 and the same stick gives 3-1/4 in each way. Same moulding, same room, layout line an inch off.
Why 38/52 and 45/45 both exist
They are the two angles the profile is designed to sit at, and the pair always adds to 90 because the wall and the ceiling meet at a right angle. A 38/52 profile leans further onto the ceiling and drops less down the wall, which is why it reads as a wider band from the floor. The names are trade convention rather than a standard, and there is nothing stopping a manufacturer shipping something else. Measure a scrap in a square if you have no data sheet.
The setting from the book and the corner in front of you
The compound pair comes out of two things: the spring angle and the corner angle. With the defaults, a square corner wants 31.62 degrees of mitre and 33.86 of bevel. Open that corner to 92 degrees, which is an entirely ordinary drywall corner, and it wants 30.73 and 33.19 — the mitre has moved 0.89 degrees.
Now leave the corner square and let the stick sit 3 degrees off its nominal spring, which happens the moment a bead of drywall compound or a bowed ceiling pushes it up. The mitre moves from 31.62 to 33.27, which is 1.65 degrees. Over the three-degree band the form uses by default the mitre swings 3.43 degrees in total, against 1.79 degrees for a corner two degrees out either way.
That is the useful thing to carry away, and the page prints both numbers so you can compare them for your own profile. How the moulding actually sits is usually a bigger source of error than the corner being out of square, and it is the one nobody measures. It is also why fitting crown is a business of cutting two offcuts and holding them in the corner rather than dialling in a number and trusting it.
Nested or flat
Cut nested and the moulding sits on the saw at the same angle it sits on the wall, upside down, with the ceiling face flat on the table and the wall face against the fence. That means the fence has to be at least as tall as the wall drop — 3-5/8 in with the defaults — and it means the saw is set to a plain mitre with no bevel. Most auxiliary fences on a job site exist for exactly this.
Cut flat and the moulding lies on its back, which needs the compound pair and enough table depth for the ceiling dimension. Wide crown that will not stand up under a fence has to be cut this way. Neither method is better; they trade fence height against having to hold two settings at once.
The backer block
A continuous nailer in the corner gives the crown something to shoot into between studs. It has to fit inside the triangle behind the moulding without touching the back, or it lifts the stick off its spring angle and every mitre in the room is then cut for an angle the moulding is not sitting at. At a 2-1/2 in block height with the defaults, the thickest it can be is 7/8 in. Cut one, hold a piece of crown against it, and look at the gap before you run fifty feet of it.
What this page does not do
It does not count sticks or footage. That take-off, with mitre waste and the scarf joints a wall longer than a stick needs, is on the trim and molding calculator. It does not handle polygons, staved cylinders or sloped work either — the general compound-angle pair for those is on the miter angle calculator, which takes a side count or a corner angle and a slope. If you are new to running a mitre saw at all, the woodworking beginners guide is the place to start.
Questions people ask
What is the wall and ceiling dimension of 4-5/8 crown?
With a 38 degree spring angle, 3-5/8 in down the wall and 2-7/8 in onto the ceiling. With a 45 degree spring the same stick gives 3-1/4 in each way. The face width alone does not tell you — the spring angle is half the answer, and it comes from the profile maker or from measuring a scrap set into a square.
What mitre and bevel does 38/52 crown need on a square corner?
31.62 degrees of mitre and 33.86 of bevel, cutting the moulding flat on its back. Cut nested against a fence instead and it is a plain 45 degree mitre with no bevel, because the moulding is holding its own spring angle on the saw. The page prints the flat-cut pair for whatever corner angle you actually measured.
How much does an out-of-square corner change the crown mitre?
Less than most people expect. At a 38 degree spring, a corner two degrees off square moves the mitre by 0.89 degrees and the bevel by 0.67. A stick that ends up sitting three degrees off its nominal spring moves the mitre by 1.65 degrees, which is nearly twice as much. The corner is the thing everyone measures and the spring is the thing that actually moves the setting.
How tall does the saw fence need to be for crown?
At least the wall drop, if you are cutting nested. That is 3-5/8 in for a 4-5/8 profile at 38 degrees and 3-1/4 in at 45 degrees. Wider crown needs a taller auxiliary fence, and past a certain width it has to be cut lying flat with the compound settings instead, which trades fence height for table depth.
Should I snap the crown line off the ceiling or off the floor?
Off the ceiling, in almost every case, because the top edge of the moulding has to stay tight to the ceiling and any gap there is the one people see. The cost is that the bottom edge picks up whatever the ceiling wanders — half an inch over a room is common. That only matters where the crown runs past something level, like the tops of tall cabinets, and then it becomes a judgement about which line the eye reads first.