Adjusting proportions and adding new objects to match sketchover:
In photoshop I set the sketchover to 50% over the original model render.
In Modo/3DS/whatever, I have two viewports set up: one for modelling, one with the cameraview that I rendered and sketched over previously. This way I can check the cameraview against the 50/50 reference image for consistency in real time. Alot more accurate than eyeballing it and easier too.
Sketchover 50% over original model render
Revised model after matchup