The thumb has to be calibrated! I've calibrated mine over the years and do have asbestos fingers. If someone has a temp gauge, read it then do the thumb test. It doesn't take long to work out if you've overdone it by using a thumb.
Gear ratios should only be checked against the clock, never a temp gauge. The right ratio is the one that goes fastest round the track, not the one giving a certain temp reading. The thumb test helps make sure that you don't wreck a motor, the clock tells you if that ratio is right. HTH
