Had not heard of the Borg Scale of Perceived exertion before.........(will resist the urge to add Star Trek or tennis connotations).
https://www.hsph.harvard.edu/nutritionsource/borg-scale/
"One way to gauge how hard you are exercising is to use the Borg Scale of Perceived Exertion. The Borg Scale takes into account your fitness level: It matches how hard you
feel you are working with numbers from 6 to 20; thus, it is a “relative” scale. The scale starts with “no feeling of exertion,” which rates a 6, and ends with “very, very hard,” which rates a 20. Moderate activities register 11 to 14 on the Borg scale (“fairly light” to “somewhat hard”), while vigorous activities usually rate a 15 or higher (“hard” to “very, very hard”). Dr. Gunnar Borg, who created the scale, set it to run from 6 to 20 as a simple way to estimate heart rate—multiplying the Borg score by 10 gives an approximate heart rate for a particular level of activity."
would be interesting to see the results of this.