i don't know exactly but believe it or not last year I was down to 16.5 inches for the last 2 weeks and no light burn at all!
How did the buds turn out with the light that close?
Your doing a great job so feel free to ignore this but I would probably drop it down to 24 inches or less now.
the buds were tasty and sticky aswell!
Then I probably wouldnt change a thing