I assume your talking about having the jambs of the new windows sit flush with the interior wall surface? so your installing completely new windows into the homes rough openings or are you doing replacement style and reusing the windows existing frames/jambs?
If doing a completely new window you will hve to get measurement for wall thickness. This cna be done a few different ways, you can goto your door and meaure jamb thickness from back side of brickmold to back side of interior casing, most jambs are 5 1/4" to 6 9/16" and old plaster homes are always difficult to get a precise reading since walls vary ALOT from one point to another. Just finished one home where there was 3/8" difference in thickness from top corner of frame to the opposite lower corner.