don't go with %HR
I would suggest looking at your actual HR and not your %. The %HR is so very misleading. At some point you will find a hill and learn what your REAL max HR is...and at some point you will (if you watch your actual HR number) learn your anaerobic threshold number when you realize that at (for instance) you can breath OK at 160bpm but at 163 you get out of breath very quickly. Once you know those two numbers you have a good gauge from which you can determine the zones you want to work in. Your garmin won't move your HR% over time even though you will get stronger and your max HR will increase, as will your AT, but YOU will see both numbers go up. My AT has moved from roughly 130 when to 160 in the last year and a half.