No experience with the exact configuration but I would guess that a 30mm will measure out to ~28mm ish on an old school narrow rim.
I went through a similar situation as I have newer wider rims on some bikes and older narrow rims on other bikes. I've found the old 25mm Conti GP4k tires on 23mm external rims to be 26-27mm, which just fits my older frames. The new GP5ks I've read run narrower due to compensating for modern wider rims.
I found that I can maybe run 28mm tires on narrow rims, or I can run 25mm tires on wider rims. But 28mm tires on wider rims for sure won't clear any of my road frames.
So I basically stick to 23-25mm tires on modern rims and 25 tires on older rims.