If you can hold your hand at the same level as the plants under your lights and your hand does not feel uncomfortably hot then your alright on light height. This is what normally causes this curl under problem. Now if your leaves are curling under like rams horns with multiple curls then this is a too much nutes problem. Also I find some under curling (single or partial loop) relatively harmless to the plant especially in the final stages of budding.

I hope this helps.

Herbus