ProAce:
What I meant is that if I'm ever going to use other fans, I will have to calibrate the formula to them.
You could program the Arduino to self calibrate. Attach a pushbutton to the Arduino. Code your sketch so that when you set all fans to 12V/2200rpm and press the button, the Arduino measures a pair of pulses from each fan and stores these calibration values in its eeprom. These values can then be read back from eeprom in setup ().
ProAce:
College is really paying of than
Sorry, don't understand.