Calorie Calculation for Circuit Training

Tatonka_usn

Quick question for the masses, and was hoping you smart folks might be able to help me out. After many moons of doing cardio training only (elliptical, treadmill walking, stair-stepper), due to back/hip issues, I finally stepped out my comfort zone and opted to try a bit of circuit training this afternoon. Unlike the cardio machines, which give you caloric feedback, life-fitness circuit equipment doen't have a similar function. As such, and given the fact I don't have a HRM at this point, I'm a bit confused as to what I should enter for myself. MFP's calcuation function provided me a general feedback of 400+ calories for roughly 40 minutes, but I was lifting VERY light (just trying to gauge how far I've fallen over the years, muscle-wise), and just don't know that I trust that number. How do other folks manage this variable of their workout, without a HRM? Any thoughts/feedback would be GREATLY appreciated!

Alan