Hill climbing, lower gear wanted?

Is that right? If so, at least for MY uni and bike, the feeling is much different. (well of course it does… but still)
I fear I might be getting into difficult math territory by asking this, but…
How do I compare my uni to my mountain bike granny gear?

I could mark a chalk line on the driveway, roll the uni on revolution and mark again. Then start my bike on the line, in granny gear, move one crank revolution and mark.
If they went the same distance, they’re equal?
Wait, no… I need to account for crank arms somehow, right?
I’m suddenly remembering a thread some heavy thread I skipped over. My brain is starting to hurt again.