Here's Jillians equation for working out your heart rate.
"I suggest working out at about 80-85% of your MHR. Here is the equation for you to figure out what that number is. First you must calculate your Maximum Heart Rate by subtracting your age from 220 if you are a woman or 226 if you are a man. Then figure out what 85% of that number is. That number should be the number of times your heart beats in a minute when exercising to get the best results."
Hope this helps!