I think with experience you will be able to quantify effort by feel, from 30 - 90% and corollate it to you cadence range. I have calculated my base cadence range using my speedometer and noting the sprocket selection to be about 60 - 90 rpm. At 40% effort 60 - 65 rpm is good, 60% about 70 -75 rpm, and at 80% effort perhaps 80-85 rpm.

Even as a recreational rider, I still like to monitor my effort by feel and cadence sometimes when fighting strong wind or climbing steep hills. Experiment with the gears. I used to use only very low gears on a hill in my neighbourhood, But found it much more efficient using taller ones for much of the ride.

Actually now that I think about it, A cadence sensor makes sense.