effort level in practice

Swimmers and coaches often set workout targets like 90% effort or 95% effort for practice swims. I've always found these directives to be less than useful. What is "90% effort"? I've taken to setting time targets of race time plus a certain percentage. For example one could specify the set: 5x(100 free @ race + 15%)/2:00. That is, five 100 free swims on the 2:00 (120 sec.) interval with a target time of race time + 15%. (This would be a painful lactate production set in my estimation.) Based on my own experience, I constructed this chart giving qualitative descriptions of the effort level associated with a practice swim from a push to achieve race time plus a percentage: 7283 A few notes: This would correspond to the effort level of the first swim in a group. Obviously even race +25% will constitute a very hard effort after you have done a lot of them. Generally, it appears to be easier to swim at race pace + x% for longer swims. It is easier to swim near race pace for backstroke than freestyle. I suspect that this is simply due to the fact that a freestyle dive start gives more of an advantage over a push than a backstroke race start. I pose the following two questions to the forum community: 1) How do these effort levels compare with your experience? I'd love to see similar charts for other swimmers. 2) How much time do you spend in practice at each effort level? This will certainly depend on the time of the season. Early in the season I expect one might do a lot of "blue" swims up to some yellow, whereas later in the season one needs to spend a lot more time in yellow with frequent excursions into in the "red zone".