Anyone know how to calibrate a 1966 Vacuum gauge in the Oldsmobile to Chevy swap?

[Tpo]

I’m bumping this because I have a similar question. I’m close to buying a beater Pontiac or olds gauge and installing it in my gauge panel on my 1960. However, the olds and Pontiac gauges read 0-24” while the correct Chevy truck gauge reads 0-30”. Plus they have different “sweeps” if that is the right term. Sooo, if I simply put a 0-30 overlay on the 0-24 gauge, what’s the deal? In his awesome thread, chevyrestoguy adjusted the needle and stops, but it was unclear if the gauge read and swept accurately afterwards.
I don’t mind keeping the stock Pontiac overlay on the gauge if it means it is more accurate, but those repro overlays sure do look nice.
Thoughts?
Thanks!