Small diameter retainers will sit down inside the recess of a std height mic.
The recess is usually .150” deep, so you would deduct .150”(or however deep the recess is) from whatever the reading on the mic is(1.800 reading is really 1.650 IH).
So, when working inside the recess........ the range for the mic has to go at least .150” taller than the actual installed height for the parts your working with.

As mentioned, you can get it done with a snap gauge and a caliper.
Even just a caliper will work okay on some heads.

I have next to zero experience with Magnum heads, so I can’t say which mic will clear the factory spring step.
If I were trying to assemble the heads and my mic didn’t fit, I’d just measure it with something else.