So I finally got a heart rate monitor, and I have a heart rate to base zones on. I'm a bit confused by Friel's table, though. The instructions are "Find your LT pulse (bold) in the "5a Zone" column. Read across, left and right, for training zones." Seems clear enough, but the 5a Zone column has a range. Do I want my LT heart rate to be in the middle of the range, or is it one of the extremes?