I was not sure how you are calculating your heart ranges....but barring getting tested on the bike, there are a few better ways to calculate HR ranges and MHR outside "220-age" -- try the Karvonen method which takes into RHR as well. See this site for a good calculator, but if you are interested, PM me, I also have the "long-hand" calculation you can do yourself.

http://briancalkins.com/HeartRate.htm

My 220-age (I am also 44) would yield 176, however, the Karvonen method plus what I have actually seen on my Polar are a little higher than that. So even using Karvonen, they don't pre-suppose an exact MHR but if you have a good idea what that is based on your own trials, you can work backwards with the percentages and use a bit of a hybrid method for your calcs.