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Rollout is just a more complete way to express a gear ratio that includes tire size in the equation. There are enough smart phone apps out there that you really don't need to know the equation.

The rollout number that is generated is how many inches the car will go for every motor rotation. Use a bigger spur gear, smaller pinion or smaller tires to get a lower number. Smaller spur gear, bigger pinion or bigger tires to get a high number. Lower rollout will produce better bottom end torque. Higher rollout will produce more top speed at the expense of the motor bogging down during acceleration.

Finding the ideal rollout that is best compromise between acceleration and top speed for your particular car and track is definitely key for getting the most overall speed and lower laptimes. But it does seem that with brushless motors there is a range of rollouts that work so you don't need to worry about getting it "perfect" down to the hundredth or even tenth of an inch.

I think you're just looking for the rollout that gives you the best balance between bottom end and top end speed for a particular track (without overheating the motor.) Lower wind motors like a lower rollout and higher wind motors need a higher rollout. For mod I usually start at 1.0 and tune from there. If I run a spec class I ask a few people who already run the class and then average their answers for a starting point. Everyone's motor is going to run a little different and driving style affects it too. I'm not aware of a "magic formula" but maybe if you had a chassis dyno you could figure something out to get it close.