If you are going to worry about that method, realize too that first your HRM calorie burn is likely inflated, up to 33% for women.
Second, that HRM reported burn includes what you would have burned doing normal activity, at least sitting or sleeping, so BMR calories for that time must be removed.
Third, your calories for every hour is actually already accounted for, your non-exercise TDEE. Could be sedentary, could be lightly active. In the world of dieting, that is how much you were going to burn per hr. So you remove that from your HRM calories.

To example provided then.

BMR 1600 x 1.25 sedentary TDEE = 2000 non-exercise TDEE / 24 = 83 calories is already expected to be burned every hr of the day.
500 calorie burn in 90 min according to HRM means 125 calories already accounted for that 1.5 hrs.
500 - 125 = 375

Your exercise actually burned 375 more calories than what was already accounted for already.

1900 - 375 = 1525 - negligible.

Considering the body's needs don't stop at midnight anyway, like it knows, any 48 hr time period that balances out is just fine.

Because really, you can find in your week some 8, or 16, or 24 hr period where you did not NET your BMR for that time period. Probably many times through the week.

And that's a huge assumption that 500 calories is dead on.

http://www.myfitnesspal.com/topics/show/459580-polar-hrm-calorie-burn-estimate-accuracy-study

As long as each and every day isn't netting below your BMR after you calculate the calorie burn correctly, there is balance within a 24 hr period likely.
One day easy workout, one day hard, one day rest, the week ends up above your BMR on avg 100-200 calories.