Beta Blockers and Exercise

When I first joined MFP I thought the kcal burns for my exercise seemed too high. I researched the forum and afterwards bought a heart rate monitor, as it seemed this would be the most accurate way to work out my kcal's from exercise. Afterwards I found out that the beta blockers I take for my heart condition (congenital heart failure) make my heart beat pretty slow and the results I was receiving from the HRM inaccurate. I normally swim breaststroke for 60 minutes non stop and almost every time manage around 1600m. Using the HRM I normally get around 300kcals burned per swim. I have found this calculator http://www.swimmingcalculator.com/swim_calories_calculator.php which is supposed to be quite accurate. Knowing what my HRM says would it be wrong to use the results from here instead? The results from the calculator seem to be a middle ground between my low HRM calculated burns and the high MFP burns.