Well it's not really a answer but I have a globee and a TNC and both give goofly readings unless I get just the right spot and oddly enough it is with the tac almost touching the cdi box so go figure
I know it's better out near the tips the tnc will work father away but it still likes the outside 1/3 of the prop
I don't know why they don't make a simple plug in tach that could go in like with the senor seem like a simple pulse counter and led display would do the trick
maybe an electron guru will make one and get rich