lower light until you feel a slight heat on the top of you hand when located right at the top of your plants. 16-18" is good usually, but If you have good fan blowing across top of plants and under the lamp, you can get it closer....the key is...you want your nodes to be closer together, anmd the closer then light the tighter nodal spacing will be and in the future...tighter buds.

A rule of thumb is: lower lamp until you cannot stand the heat from lamp after 30 seconds...then raise it 3" and check again...Hand at top of canopy...too hot! raise light.................Not hot! lower light. Now take note of distance from plant canopy and keep it there; raising it as necessary to keep lamp at safe distance...
remember...for every foot you have your lamp above cnaopy...You are losing 1/2 the light's potential intensity on the plants. peace