If you take a guideline- then 220 less your age is a good place to start. If you want to get serious- then a stress test should be the next step after consultation with the doctor or medical experts.

I had a bypass 10 years ago and Had a stress test a year later. This was as near as dammit- the same as the 220-age. I treat my max as 160 at 63 now and occasionally reach it but that has to be after a lot of exertion and a very good reason to do it. I also have to be in peak fitness to get there so after a long winter- I will not be hitting it shortly.

But there is a good Non-Technical way to see how hard you are working. If you are breathing hard and can only get out short sentences to a rider by the side of you- Then you are probably about 75 to 80% of your max. IF Those---Sentences--- have to---be broken---for breaths, then you are probably around 85 to 90% of your max. And if you have to get off the bike and lie down before you fall down- Then you have gone over the top.