as you are growing with a36 w fl-light you are able to go quite close
you wrote 20 cm with the light and i will agree with that, if topleaves get bleached or yellow, it is to close
if plant is dwarfing ( growing with extra short internodes) its to close
if she grows with internodial spaceing for arounnd 1,5-3 cm its good
latger distance, it is to weak
do you have another light for flower=?
because the 35 fl wount be enough