All rollout is the amount the car moves forward (rear tires roll) for one revolution of the motor shaft. Rollout only takes into account gear ratio and tire size (circumference). Since it is much easier to measure tire diameter with calipers, everyone uses that.
I posted an Excel spreadsheet in the BMI DB12R thread that allows you to easily find your rollout.
See this post:
http://www.rctech.net/forum/4792583-post5349.html
For a given spur size, the left hand column is pinion size, the top row is tire diameter, and the rest of the data is the rollout. So, after entering your spur size, measure your tire diameter, go down that column until you find your desired rollout, and the go across to find the pinion you should use. The top chart is rollouts in mm, the bottom is rollouts in inches. Both tables have the tire diameter listed in mm and inches, so it doesn't matter what units someone else gives you the rollout in, you can easily find it.