Long answer.
For pistols, I trace the pistol, measure the width of the slide. 1/2 of that plus 1/2 that of the leather used. Allow for mag release on the back ect. Shorten just a tad at the lower end of the trigger guard. This is for a pancake holster. 1 inch to the inside of the belt loops. The stitch line stays the same for a drop holster.
For a chest holster, I figure the balance point and place attachment points accordingly.
For a revolver.
There was a very good video of this and if I find it I will attach it.
I build up behind the top of the front sight to the height of the sight to the back sight. I use thin plywood or a dowel rod, tape it down. Then trace the gun.
I take 2 narrow pieces of leather the same thickness I'm using for the holster, and connect them at one end. Rivet or stitch.
Draw a mark where they connect when parted. That becomes your line of reference at the sight line.
Hold this along the sight line and starting at the trigger guard, make a mark where the leather touches.
Proceed down the revolver every place it narrows to the end of the gun.
Go to your tracing and transfer your marks. Placing the sight line mark on the sightline working from top to bottom.
This will be your stitch line.
I believe it was Adam's leatherwork that did the revolver video.
Just my 2 cents worth.