Lights aren't my area of experise, but coming from an horticultural background I'm a bit familiar with commercial greenhouse setups. Those growers generally have their lights set up to a sensor, and everything turns on (or off) at a preset lux threshold. The point is to insure a minimum light level, maximum isn't as important.
As to 'yield' and 'time to harvest', sounds like you aren't growing orchids! I'm thinking of a certain other plant that loves to grow under lots of lights...lol