For a single layer of leather, I wouldn't add any extra. When I make a double layered holster, I add twice the thickness of the leather along the fold line of the holster.
The outside edge, correct?
If you have any doubt about a pattern, try making a holster out of cardboard first to make sure you have it right.
I'm doing that now with a pattern I made, and the cardboard pattern was cut 3/4" from the outline and fits the gun ok, but even if the cardboard fits, the holster can end up being too tight or loose depending on the thickness of leather I use.
The way I'm understaning your suggestion is to draw the outline 3/4" from the outline of the gun and then add twice the distance of whichever thickness of leather I chose. For example, for a 14 oz combined thickness trace a line 1" from the outline (3/4" plus 7/32" (14oz leather=7/32" or roughly 1/4" ).
Am I on the right track here?