Dunno, I would use the method I described in a similar post the other
day.
Use an average meter, and talk and adjust until you get to a point
you see no visible increase on the wattmeter. A stock mike should be
at about the half way point of the mike gain. If you can go all the
way up, and still see increases, you may need a preamp. Not familiar
with the yeasu mike setups...My 706 needs one using my audio technica
mike, but fairly normal for the icoms. A heil needs a preamp to have
enough drive on the 706. Might need one on the ft-100, but I'm not
sure. Compare it with the stock mike, and see if you see a large
difference in mike drive. MK