Making the transition from western to concealment is not to bad as long as you rethink some things. The biggest difference is the stitch lines. Western holsters, one pattern may fit most every western gun with the same pattern and the same stitch line. The concealment holsters the stitching done around the trigger area and front of the gun has to be exact for each and every model. The tolerances are so close that just a fraction off will make an unusable holsters.
Western holsters hold the gun and sometimes a looser fit is wanted to speed the draw. Concealment we match security of holding the gun with speed and comfort. The average or norm for the stitch line is half of the gun width and half of the leather width from the gun. This is just a place to start as many other things will affect the measurements.
The next big thing is the center of gravity. This affects where on the belt the holster can be put, type of holster that can be made, the weight of leather to be used.
These are just a few of the main differences. On my web site on the last page is a PDF that shows some of the steps that I use. There is a gentleman on one of the other forms that has some some great learning videos that I highly recommend. Check out these at
http://www.adamsleatherworks.com/