Gauging effort expended based on HR

Does anyone know how to gauge the percentage of effort expended in relationship to your HR? We had a set yesterday morning that got me thinking, and I am really having trouble wrapping my brain around this one. If your fastest time in the 100 free is 1:00, then what should 60%, 70%, 80%, 90% effort be (time wise?)? And also, since I know that your maximal HR should be (theoretically) 220 minus your age, at what HR are you going certain percentages of effort? I was swimming 5 x 100's on the 1:30 & going what I thought was about 60% effort and holding 1:16-1:18 - which was slower than I intended but the girl in front of me was going even slower, so I had to pull back - HR was about 110. Second set was to be of course, faster/harder (coach said 1st 5 HR 140ish, 2nd 5 HR 160ish - for the 15 yr olds) I held about 1:14's & my HR got near to 140, I think - but my effort felt closer to 80%. Gauging your effort is HARD! - thus I think if there was a nice little mathematical formula it would help! The only problem is, I KNOW that often an increase in effort often only results in a second or two drop/100. So, any genius's out there who can help me out?