I fine tuned it by eye 
no seriously I just went outside and adjusted the beam until it “looked good”.
it’s a 230mm diameter and focal length =190mm.
I took that in account when I designed the cone, and it slides up or down on 3 rubber O-rings which seat themselves inside the 3 heatsink fins of the C8+/M21A head. So the focus is fully adjustable.
Both have some interesting replies on how to achieve better numbers for V2.0. The link for the lens is also in the BLF thread.
The next build (3x21D) will hopefully also include a wavien collar photonic recoil harness.