What is the ideal height?
I would say you have found it.
If the fixture was too far away you would not have the compact, tight, bushy internode spacing, that your plants are displaying.
,,,If they stretch out like awkward teenagers =
Too far
,,,If they leather up, shy away, or wilt down =
Too close
Those went into foxfarm product ?
I have never used it, although I hear that some mixes arrive 'hot' Meaning non-inert.
Did the purple pot get the same soil, @ the same time ? If so , that little guy could be your indicator gauge ( like the canary in the mineshaft).
At the moment the smallest plant looks like it would appreciate a healthy breakfast.
Use the smaller plant as your Goldilocks tester.
On a smaller plant like that -if the soil is too hot, you will see burn from the leaf edge, inward.
"This one is to hot"
If the small plant stays pale, or displays new growth (even) lighter in colour -
"This one is too cold !"
If it greens up, and kicks ass.
"This one is just right"
She will be in your bed in no time !