Remember - when you make a headboard it has to fit your elevator.
It also has to make the turns in your hallways.
If you are heading straight down a hall and have to make a right angle turn - it has to make the turn.
Most headboard have legs only 18- 22".
If the legs get too long - there is too much weight on top for the skinny long legs.
Actually if the legs are shorter - even if there is a larger headboard - it's less likely to split - and less movement of the headboard.
If they need to attached to a bed frame with screws - it should be done on site at the factory - where accurate measurements are made. You can take the measurements - but no guaranteeing that it's right on. A fraction of an inch can make the difference.
It's best to have the headboard below the top mattress. The weight of that top mattress will help hold the headboard against the wall stopping it from falling forward and/or shaking. Which will drive you nuts!
If the headboard is not bolted onto the frame - then the more support against it is best. But that means a larger headboard body. So you have to watch how high the headboard will go.
Here's one with the headboard at the height of the top mattress. I don't like the look of seeing the cut off point of the headboard. My guess is that this headboard is actually hanging on the wall by a 2x4 support and not using legs.
If the top mattress is 12" deep - this headboard is at least 6" below the top. This is a good example of where the entire bed is supporting the headboard - by using the top mattress.
Pottery Barn makes all their headboard almost to the ground for full support. He can't do this height and these legs and get into an elevator.
Nice enough - they have given us measurements for the drill holes. But the distance between is a half inch different.