That's a pretty open-ended request. =)
I'd suggest looking at it like a balance, mass, energy, financial, whatever makes most sense to you.
Rage spent always has to be less than or equal to Rage generated. Rage generated is a function of white hits, so hit/miss/parried/dodged numbers are important, as is weapon speed, haste, etc.
Rage spent is a little trickier. Rage generation will be pulsatile (comes in chunks, not a smooth/constant increase), but regular. Spending is similarly pulsatile, but comes in bigger chunks. You can probably get plenty far making average numbers and not worrying too much about the specifics of pulse timing.
So, input as a function of the factors that will improve or reduce it (hit, expertise, haste, etc).
Output as a function of what moves you use, when, and probably with conditional filler like HS.
May also be worth your while to calculate the damage/threat-value of rage. That is, I spend 5 rage on this ability to get this much threat, or 20 rage on this ability to get this much threat. Accounting for differing scenarios, number of opponents, etc.
Don't forget to eventually include raid buffs as they will be important for swing speed, and will bring you closer to the pertinent conditions most players see.