Say you're doing a brevet series: 100 - 200 - 300 - 400 - 600k. I find it just the thing to set my HR limit for each brevet at the maximum HR I was able to attain at the end of the previous brevet. So, going off MHR, no limit on the 100k, 90% or so on the 200k, 88% on the 300, 85% on the 400, 82% on the 600, for an example. Cruising HR as seems reasonable, say 65-78%.