LED's and distance

[Gingerbeard]

It seems my Viparspectra 350 watt LED is unique in the forum. I see 300 and 400 watt but not much in the middle. I'm flowering 16"-18" between the V and tops of the tallest plants during flowering.
Where is the distance measured from? Light to soil or light to plant tops? Maybe somewhere in the middle?

 

[Toker1]

Most distances from LED manufacturers are talking from light source to canopy.