Stop worrying about motor stick length, there is NO reason to match it to rubber length!!!! It's very common for top level indoor competitors to fly very successfully with motors 2 to 4 times the length between the hooks. It just dosn't matter.
Reason, as soon as you wind your motor a few turns it will shorten drastically and be in tension on any motor stick. And these are the weakest turns contributing least to length of flight.
About the only thing you don't want, and even this isn't very important, is to have a lot of stretch on the motor before winding. There you've already stored energy into the rubber that you can't get back, no matter what. And any given band only stores so much energy.
About the only thing the motor stick length affects is stability. Too short and the copter tends to wobble more. But any reasonable length works, unlike Wright Stuff or Gliders where fuselage length plays a key role in balance and stability.
So, just pick a length build your copter and start testing.
Oh, an alternative to the test plan above. Instead of motor length you can use motor mass. This will be slightly more independant than length, but either can work.
Hmm, you ask how to vary the motor length. You are using competition rubber like TAN Sport or TAN Super Sport, right? The stuff that comes in continuous strips that you tie in a loop? You just tie the loop however long you need it and cut off the excess tails from the knot.
If you are using office rubber bands, you are WAY behind the curve, they simply do not store enough energy for this event.
Quick comment on breaking your bands. If you intend to win you will break a lot of motors. Get enough rubber to not be worried about winding and breaking. Turns are fuel in the tank, if you aren't winding to close to breaking, your are entering an endurance race with half the fuel in your tank.
Another comment on breaking motors. If you wind to torque you will find that a given width WILL break consistently at a fairly consistent torque. Turns are a very poor predictor of breaking by comparison.
Finally, for now, look back on this string, or last years helicopter discussion for discussion on the importance of winding to torque instead of turns and about the importance of taking data on your motors so you understand how they store and release energy.