It sounds like too close. So close is good for stopping the plant from growing upwards and making the lower parts get thicker. Assuming a typical 120 degree UFO, keep it about 3-6 inches from the top.

The plants will respond almost immediately to a change in light, and if the source is good it will respond quite favorably without the need for any supplementary additions.

The deep blue will really stunt growth up close, which is one reason I didn't include it in my custom panel. I like the little extra stretch for flowering room.

White LEDs suck, they're typically blue or UV diodes with phosphors on top. You lose efficiency with that phosphor layer by downshifting to different bands, and wasting energy in the green band.