Malcolm,
in the final analysis, it depends, at the low shore values, how much engine movement you get on and off the throttle, and if it starts knocking on the bulkhead when you brake or blip the throttle etc.
At the higher shore values, general vibration transmission into the chassis increases and can excite chassis resonances at certain speeds. Also the engine mounted natural frequency which would normally be below tickover, might come up to tickover or into the running speed range, in which case there would be a lot of engine vibration at one or two particular speeds.
I agree and would rather start at 70, 83 could be too hard. Nothing wrong with experimentation though.
regards
Dave