I agree with JimB- After a while you know exactly where you are... also, fwiw, those guidelines for calculated MHR are mostly junk...you can get your MHR much higher than you think- I'm 52 and have gotten it jacked over 190 on many occasions-  in your calculations try to actually achieve your MHR at least once for a benchmark; then subtract out your sleeping/resting HR (RHR) from that to get your working HR zone (WHRZ)... so when you go back to figure your 60% effort for example you do that on your WHRZ and then add back your RHR- makes a huge difference for me on my rower (cardio) I'm up to 130 by minute one; get up to 160 by 5 minutes; gradually climb past 170 by 10 minutes; and generally peak at 185 by 15 minutes then gradually work back down the longer/better you get used to working at your aerobic threshold (about 85% MHR or above 170) the better in shape you'll get other tip: you really want to go after it hard, max. OT if you can...while you'll burn calories/fat during any type of workout, if you do it harder and harder, you'll burn many more calories and fat during the rest of the day (aka, raising your metabolism); if you settle for less stressful efforts the lingering afterburn will be much lower. the key these days is to go hard/fast w/max effort for about min. 15 minutes or so; if you can squeeze out 18-20 that's fine too; but anything over 20 minutes IMO means you're going too slow! enjoy the pain