Mark,

On highways the way incline is measured it is not what one would think, or at least what I would have thought. Instead of "rise over run", the standard mathematical concept of slope, it is "rise over distance travelled on the road". On a highway a 10% grade rises 10 feet for every 100 feet of road. To a mathematician that would be about a 10.05% grade. The two numbers are close enough. Your calculation is consistent with the way percent incline is measured on roads, assuming your treadmill measures incline in the same way. Even if it doesn't, the estimate is so close that you don't need to worry about the difference. Enjoy your climb!

Victoria