Take this for what it's worth, I'm a novice, but what I've done to obtain my stitch lines is use a caliper to determine the thickness of the widest portion of the slide of the weapon. Take the measurement and divide it in half, then add 1/8". Do the same thing for the trigger guard. Once I have those adjusted measurements, I use a compass set to those measurements to use as a guide. Then break out the straight edge and make your final mark on your pattern. I may be doing it wrong, but I've had some really nice seams as a result of this process.
For example, I'm working on a Glock 22 IWB at the moment. Slide thickness is roughly 1". Divided by 2 is 1/2", add your 1/8" seam allowance making the distance from weapon to seam 5/8". The trigger guard is 5/8", divided in half is 5/16", added 1/8" makes distance to seam 7/16". I wish I could find the page that I saw that on, I would throw the link up here.