Diver Daver has spoken with absolute wisdom. Keep in mind that many betablockers prevent you from going past certain heart rate, regardless of how hard you work. In this instance, it is wise to disregard the typical heart rate formula (which is a flawed concept anyway) and go more by how hard the work is (this is known as Perceived Rate of Excertion).
PREs come in two popular scales one from 1 to 10 and the other from 6 to 20. Whichever is used is up to the recommendation of your doctor and the judgment of your exercise physiologist/counselor.
Again, Diver Daver emphasized speaking with your doctor, the side effects, and the approach that should be taken.
The good news is that with continuous exercise, you may need less of the prescription in the future, and eventually not need it at all. I have seen many people achieve that.
Good luck and happy diving