It's very rare that you need a boat to go as fast astern as forwards, ( no jokes about the Italian navy please ) so some
controllers have reduced speed astern (eg, Ripmax Sea Rover, only 50% astern), they say to stop the stern from being
over whelmed but think it saves on Power Transistors....!
The "calibration function" maybe like a 'reset' so different controllers can be used at a later date. I've also used it to
limit the "throw" of the transmitter stick so that I get full throttle at say 80% of stick movement.
You'll be very lucky to get two motors starting and spinning at the same speed throughout the throttle range, motors,
couplings alignment, shaft friction, props, all create variables and will effect the final prop revolutions but as 'Sir Dicky of D'
says the Action combined mixer and dual speed controller unit - P94 goes a long way to reduce the effect.