That's pretty much how I do it- guess how much is low, moderate or high. I just got the hand-me-down heart rate monitor from my dh when he upgraded. It tells me how many calories I've burned. 100 calories = 1 activity point. I never actually use any of my activity points, but I do use up at least half of the weekly points now that I am cycling. It makes me feel happy to see all those unused activity points accumulating though!
I also found a site where you can calculate the calories if you want to check it out:
http://www.geocities.com/SiliconVall...alcApplet.html
Sharon