Dimly recalling some stuff from a mandatory physical education class in college, this sounds like the OP is referring to pulse-rates when exercising. MHR, I think, was maximum heart rate. That's the upper limit you can stress your heart during exercise without risking severe damage.
You're supposed to exercise at a target heart rate of something like 75% of your MHR. (Don't quote me here, Google it for exact numbers.)
One way to calculate your maximum heart rate is to take 220 minus your age, although there are other methods.
I don't think the OP's sentence makes sense - you shouldn't be racing at your MHR. It's unclear also if the 20 point difference is higher or lower than the MHR.
The second sentence stating that he prefers the 220 minus age formula is also something of a non-sequitur.