Perception of effort in workouts

When posting my workouts in Attackpoint you're asked to rate your workout from1 to 5, 5 being the hardest. The plan was to run this morning doing 3x1.5 mi with half mile recoveries - at tempo. Years ago we sold a textbook inthe Union Bookstore, "Physics for Poets" - a text for non science majors, of course. I wonder if there are any real measurable standards to determine the workout effort. My formula is a 3 is for a training run, a 5 for a race and a 1 for warmups, or walking around an airport at a brisk pace. This morning, the "at tempo" is new enough to me in that I had to think a pace to keep than just "go out and run." The effort could have been more a 4; but there's the recovery which should rate a 1. So, it was 3 workout on my log. That's the poet's measurement.