Your change in power to keep the same cadence in a different gear is for all intents and purposes just the difference between gear ratios. If youre keeping the same rear cog on, it's just ((teeth on new chainring) / (teeth on old chainring)) * (power on old chainring at desired cadence). You'll still have to figure out your power gear and assuming your workouts dictate a different cadence than what you used to determine your power gear, you will need to graph a power / cadence curve for your power gear. Hope that helps.