There are a few things that I have found to help get a pattern right the first time, the first holster I made I couldn't even get the gun in after I sewed it up.
First I try to draw up a pattern over a few days. It always seems like when I walk away for awhile and come back I see something I want to change just a little. Seems like if I do it in stages the overall finished product comes out better.
Then when I have it where I think it is right, including the stitch lines I make a copy and cut it out and spray glue it to a piece of corrugated cardboard(pretty much mimics the thickness of leather)(I can't take credit for that tip I found it on here). Now wrap the cardboard with your pattern around your object and see if the stitch lines fall where they should, you can tweak it a little if they are off. This step can save a lot of leather, especially with holsters.
I am only a little over a year back into this addiction called leather work. I have learned more on this forum than anywhere else. All I can say is there are some real masters of the craft here both the carving side and the holster/case side.
Todd