If your gym does a sub-max heart rate test, it might be well worth your while to have it done to get a better feeling for where you should be working. There are many different ways to "estimate" your maximum heart rate, but they are all just that...estimates. However, listen to Eden as far as the age-predicted heart rate measurments. They fit...maybe 1/3 of the population.
If you don't want to go through the sub-max testing try reading some of the different ways of determining heart rates here:
http://www.heartzones.com/
In fact, Sally Edwards and Sally Reed have a book out that is a good read and gives a lot of different drills for improving your riding:
"The Heart Rate Monitor Book for Outdoor and Indoor Cyclists"
Vertically challenged, but expanding my horizons.