I use Weight Watchers E-Tools... especially the iPhone app WHICH IS AWESOME BTW!!! And there's an activity calculator on there... I figure my intensity as this:
low - doing something, but no major heart rate elevation, not breaking sweat, no increase to my respiratory rate (stroll with my honey or something, or a leisurely (slow) bike ride with him on flat ground).
moderate - definitely moving, heart rate elevated, breaking a sweat (power walk, aerobics tape at home).
high - pushing my limits, huffing & puffing, sweating, heart rate elevated, "I THINK I MIGHT DIE!"
And, it goes by your weight, cuz you're burning more calories the heavier your weight, so you really can't go by a canned "points per hour", I reckon. I mean, it's not a bad way to do it, but you may be off a bit. For me today at my weight, I did 50 min of moderate intensity exercise and I earned 3 points. If I was at my goal weight, that might have only been 2 points, or likewise if I was 100 pounds heavier, it might have been 4 or 5 points.
But, if you don't have an activity points calculator, I suppose the "points/hour" method isn't a bad guideline.
Best of luck to you!