Best way to calculate calories burned.

A heart rate monitor is the best way to determine.

For guessing I'd choose circuit training I guess.

I agree. It is worth having a heart rate monitor. I find that "aerobics" under mfp is usually a pretty close match to my hrm (a tad high but much closer than any other entry).

I was just searching the same thing and I read from different people online that for level 1 that it burns between 150-190 calories.

I get much higher than that, 220 at least all the way up to 250. It varies depending on the persons height, age, weight, sex.

I always use a calories per minute calculation but I'm sure it varies by weight. I use a treadmill to get a baseline since it gives me calories per hour. I just try to guess how hard I'm working on average as compared to a slow walk, a medium walk, a fast walk, a walk with various inclines, a slow run, a medium run, or a fast run. It's a guess but as long as you guess low, it should be close enough.

And I never miss any opportunity to add calories burned. Cutting grass, doing yardwork, cleaning, painting ... I track it all. If I'm going to pick a sedentary lifestyle calorie limit, I'm going to track anything that isn't sedentary. Not the best advice but it's worked for me.

Get a heart rate monitor