Yeah, it looks like that's accurate Rob.
"The distance ranging rheostat of the ASP-3N gun sight is tuned for the NR-23 automatic cannon." - taken from pg. 142 of the full manual. So it looks like when you get your reticle set and on target, it would be for the 23 mm. So then you would have to manually compensate for the heavier 37 mm rounds.
I don't however know in practice, as I'm still just trying to get comfortable with the basics of the mig-15 so far. ^_^