Distance from collet to table

[Jonas]

for the very best stiffness you should have the least amount of stick out as possible

Yes.
And thank you for the heads up with the coating.

If the flute does not have coating and could even saw it off if it does not fit all the way into the spindle then maybe for good stiffness maybe should design it to go down to something like 10 mm. 3 mm for stock and then 7 mm between stock and collet nut.
In the picture it is 22 mm, I have to extend the face plate 12 mm down and move the motor holders 12 mm down. My guess is that face plate (20 mm aluminum) is better to extend than move motor down?