"You mean percentage of opacity.... or maybe percentage of flow?"

Well,  look at it as percentage of opacity if you like. It's really percentage of effect. Almost all the tools have a slider up top. Like, if you use the burn tool, set the slider at 50% and you get 50% of what the tool does at full use. I find it very useful to set the effect % down. If I want a stronger effect, I just go over the area again.

Talent is God's gift to you. Using it is your gift to God.