Halide height on shallow sps tank

[Proteus]

I'm looking for advise from halide experts on height on shallow tank

My tank is 12" deep painted black on two sides with black substrate. Acros range from 6" to 10" water depth. With a 250w 21k geissmann halide housed in a lumenarc mini stealth pendant. The light is around 10" from water surface. Photo period is 6 hours

Most acros are showing good color but some are still lacking.

Wondering how low I could set the light or if I should increase photo period.

I started at 14" above water surface lowering 2" every couple weeks