Ed,
When you calibrate sensitivity you pick your own screen height from the 1/16" hole, say 50%FSH. The dB level here is your "b".
When you find an indication max it out and adjust the gain to bring the indication to 50%FSH. The dB level at this point is your "a".
So "a" is your indication and "b" is your reference sensitiviy level.
Mankenberg