It seems to me that having both motors on for X milliseconds would equal the robot traveling Y distance.
This however is not the case because as I'm sure you know. Voltage and Amperage tapers off the battery as it's used (See, I learned that!)so what do I use or what term do I google to find out how to prevent this from happening.
What I want is that if the battery is supplying 9v at the beginning of its life, the motors get 7v.If the battery is supplying 6.999999v the whole thing stops.(Note: I may be talking about Amperage too, I don't know)
Essentially, I want to get as close to accurately predicting Xtime = Ydistance
I also should mention that if its possible I'd like to keep a single power supply (Arduino & Motor Controller)