Consider a Glock with a slide width of 25mm and a triggerguard width of 16mm; the outside piece of leather will be around 41mm longer than the inside piece of leather in a fully offset holster. My pancake design puts the outside piece of leather 10mm longer than the inside. This does several things. Like a traditional pancake, there is a natural sight channel and the belt slots pull the pistol in tight to the body. Like a fully offset holster, one side is still longer, making the holster stay open when the pistol is withdrawn. The outer piece can be reinforced with a piece of leather to further aid the holster in retaining its shape once the pistol is withdrawn.
The downside is that it adds a step to construction. The traditional pancake holster can be glued and sewn all at once. For an offset pancake, I glue and sew the triggerguard and trailing edge; then I insert a large dowel, wet the outside piece of leather, and glue and sew the leading edge.
I have experimented with several different lengths of offset and settled on 10mm. I create the traditional pancake pattern, then trace that to another piece of paper (I use manilla folders). I mark a line 5mm forward of the front stitch line and use that to retrace the front of the holster. This gives me the pattern for the outside piece. Then I do the same thing, except 5mm shorter. This gives me the pattern for the inner piece.