Excellent question. Comparison of climb rates are meaningless without knowing the grade and condition of the terrain. The important thing is to measure your own progress over time by using the same type of test.
As you described, 40% is too steep. For mountain sports, I would use 20-25%.
Also note that the speed control on the NT incline trainers sucks (as it does on most treadmills). The speed that is displayed is probably not the speed of the belt, so your actual calculated climb rate will be different. My x11i runs +19% over the displayed speed and Scott J.’s runs +12%. (The good news is that when you use an accurate treadmill at the speeds you’re used to, you’ll feel like a rock star.)
You have a few choices:
1. Although likely inaccurate to real life, assume that your treadmill runs the same speed when it’s at X% incline. That way your testing will be consistent even if your knowledge isn’t accurate. (My x11i varied between 16-19% faster than the display.)
2. Calibrate it. This is what I used to do; or
3. Try out a Runn. That way you can ignore the display and go by the speed the Runn broadcasts to your watch. I think this is what Scott J. has done, and it’s what I will do the next time I have my own treadmill. (More info here from DC Rainmaker)