That "220 minus age" formula really is a very, very rough guide. People who are in good shape will be higher than that; people not in such good shape will be lower. A more effective way of measuring your heart rate is perceived rate of exerction (which it sounds like you were doing with your trainer). The most accurate rate of all is to go to a local gym that does O2 metabolic assessment profiles. They'll have you strap a mask on your face and then run on a treadmill (or use a stationary bike) and will measure your breath output for levels of CO2 and will be able to tell you exactly what your max heart rate is, how many calories you burn per minute at different levels, whether you are burning carb calories or fat calories, etc.