Convertor gurus enter

Calculate slippage first. You'll need tire diameter , gear ratio , r.p.m. and speed.
Using simple math and a time slip will tell you if your converter is up to snuff.
Numbers around 10 % are around where you want to be at , and understand that this is a rule of thumb.
If it is slipping on the big end , it's probably not launching you either.
If you can't get a time slip , you need an accurate speed reading and you want it under load.......maybe bribe a law enforcement officer with a dozen crullers ? :D