Yes, MHR calculations are pretty faulty. I think mine comes out to 177 for my age and I have hit 189 on more than one occasion. Pick a point when you feel good and push hard, maybe going up a hill and just keep adjusting your max upward until you no longer are able to exceed it. Of course you should make sure by having a physical or discussing your training with your physician that pushing to your MHR is a OK for you. Now with that said most experts don't train against MHR but against your Lactate Threshold. This is the point where your circulatory system can no longer remove Lactate as fast as you are producing it and it starts to build up in your muscles causing fatigue. For me that is about 87% of my MHR. By staying below the LTH you can ride longer and by pushing your LTH you can increase it. At least that is my understanding