1) Percentage of power does have application to some engine limits. For example both Continental and Lycoming do not recommend leaning below 75%of maximum rated power.
Should be "above" 75% power.
Remember too that it may not be possible to achieve 75% rated power at altitude in a normally aspirated aeroplane so therefore you may be better off leaving the engine leaned while climbing at altitude. Some high altitude airports (not in the UK
) and you will need to lean on the ground before take off.