I was doing some research today, and stumbled across this very interesting study regarding yoga and cardiovascular fitness: www.biomedcentral.com/1472-6882/7/40
Basically, the conclusion is you burn about the same amount doing yoga that you would walking on the treadmill at 2 MPH... so not cardio. Yoga does provide health benefits, but it's just not a big calorie burner and doesn't improve cardiovascular fitness.
I think your 75 calorie estimate is probably about right, and shouldn't be an overestimate for your purposes. :)