Heartrate and butterfly training

I'm wondering if it is possible to use heartrate as an indicator of whether one is ready for longer distances, specifically in training butterfly. Sometimes I'm not sure whether I "can't" do that extra 25 because I'm mentally wimpy or physically wimpy. If my goal is to work my way up to doing a 200m fly, is there a target range I should be trying to stay in during repeats? If I've calculated my max heart rate, is there a percentage of max that I should stay under, and a percent of max at end of a repeat that says I should be able to add another 25m to my repeats? Is there a percentage of max that I can use to figure out when I'm ready for the next repeat? Training butterfly seems to be different from other strokes like freestyle in that I can't just slow down, beyond a point anyway. Even at the slowest I can go I can't stay aerobic, so it is a matter of adjusting repeat distance and rest interval. I also mix in one arm or body dolphins to increase the distance/duration I can do, and of course I am constantly trying to improve technique. Is heart rate monitoring useful in this sort of training?