Max degrees/grade/percent incline of my treadmill?

Hi, after reading lots of info on the internet, I'm still confused about this. What I do know is that at the gym, the treadmill goes up to 15%. Of what, I don't know. But it feels strenuous after a time.

My home treadmill goes up to "22%" - however it is alot less steep than in the gym!

To properly compare, I got my iphone and measured the actual degree tilt of both with an app. Sure enough, it was more at the gym, by a few extra degrees. But in order to reasonably accurately calculate for mine using the calculators online, I'm always confused about what exactly the grade% or degrees% is. I only know that the incline on my treadmill is 0 degrees off the treadmill on the flat floor, and 5.5 degrees when it is fully tilted. (and reporting 22 incline on the LCD at this point).

I've tried to use this calculator:
http://www.calculatorpro.com/calculator/running-calories-burned-calculator/

and this calculator:
http://42.195km.net/e/treadsim/

But I really don't know what to enter for the max incline for either. And I also noted on the internet somewhere that 1% grade = .57 degrees. So does that mean the max grade of my treadmill is (5.5 degrees/ 0.57)? Which is grade max 9.65… And my treadmill reads 22.

See… I'm lost. Many many thanks!

My treadmill is:
http://www.branxfitness.co.uk/buy-treadmills-online/buy-electirc-treadmills-online/branxfitness-semi-commercial-gym-runner-pro-treadmill But the LCD reads 22% at max tilt.

You don't need to be so accurate with the incline, because it doesn't make that much of a difference. Just do whatever is enjoyable, that matters far more for the long term success of your exercise program and life. Keeping it enjoyable includes not stressing yourself out about the little things.