White channels are where you need to be careful as they produce a ton of par. I agree that whites can cook corals. This is kinda where I come back to recommending a par meter, whether you borrow it or rent it. With LED's, I will always recommend using a par meter as the human eye is a very very poor judgement of par, and this is where most reefers get into trouble with leds. 20 percent may be low for white light, but not using a par meter you will just have to watch your corals. I will say after 10 to 15 percent raise of white light over a few weeks or so, I would step back and let your corals adjust for a week or two and see how they are reacting.