Proofmeter Cable RPM

The Story: I finished rebuilding the engine on my 52 8N and installed a new cable on the proofmeter. The proofmeter looks like an original and probably hasn't had a working cable on it in years (maybe decades). The engine runs fine but the proofmeter seems to be indicating higher than actual engine RPM. I have adjusted the engine idle to where it just barely turns over and the indicator shows 900 RPM. I hooked a mechanic's electronic RPM meter to a spark plug wire and I get eratic readings from 27,000 to 0 RPM (Yes, I mean 27 Thousand not Hundred). The mechanic says his meter works fine on modern cars but the spark strength in the old tractor is just too different to give an accurate reading. My idea is to hook an electric drill, of known RPM, to the proofmeter cable and see what the proofmeter reads. What I don't know is, does one RPM of the tractor engine equal one RPM on the proofmeter cable. It wouldn't suprise me if the gearing in the govenor and various gears reduces the actual cable RPM and the proofmeter is calibrated/marked to show engine RPM and not actual cable RPM. Or... I could just skip all of the investigation and proofmeter rebuild fun and buy a new proofmeter. But Hey, this is a hobby, not a job. I like tinkering with stuff. The object is to have fun while learning something, not to spend money. Besides, the tractor runs fine no matter what the proofmeter says.