When should I start bloom?

So I’m at a bit of a dilemma as this is my first cabinet grow. Currently I’m growing gorilla glue and ak47.

I have been doing LST with both plants.

With that I have about 40” left of height space for my plants to grow. That is with raising the charcoal filter and the lights raised to below the filter with a 2” gap.

Mars hydro stats for bloom the light should be 10-16” away.

So bring that 40” down to an even 35”.

when would you start bloom cycle? Say when my plants grow another 15”?

The general rule of thumb is it will grow double in size in flower so start flower when the plant is about half the size you have available maybe a little less.