I’ve never heard of a formula specific to people taking beta blockers. It’s possible that such a thing might exist, but seems unlikely to me. Beta blockers don’t just slow the pace of your heart, they decrease the ability of your heart to respond to exertion, decreasing the ratio of maximal heart rate to normal heart rate. This effect makes predicting a maximal heart rate unreliable, at best. If you are taking a beta blocker and have been cleared to exercise, the best thing to do is ask your doctor what level of exercise is appropriate for you. Using maximal heart rates for such a purpose is probably not the best idea when taking beta blockers.