RPM to PWM is one way, although it will not take load into account (i.e. 2500 rpms is a world of difference at WOT and cruise throttle). I plan on incorporating a PWM duty cycle that can be tied in to actual injector pulse width.
I hope that the final product will fulfill this for you, please be patient though as it's going to take A LOT of development and testing.
Russ.