Guy how short can we go? I saw some freestyle boards from Flikka, Starboard at 205-207cm. Can we go shorter and what are the limits and why?

Shorter means less inertia and a lower chance of the nose touching the water in power moves. If you're working on Paskos and double Air Culos, that matters. The drawback is that slightly lower directional stability and more effort to get going - which does not matter much for freestylers with good skills.
On the freeride side of things, board lengths have gone a bit the other way, getting longer again, since the drawbacks matter to people with average skills who don't care what a double Air Culo is.

Also you need a certain amount of rocker. Once you go too short, the entry rocker inhibits planing
Finding that with experimental freestyle kiteboards I've been doing, getting down past 4'4" is hard

I guess I'm looking for more numbers. Does it mean that somewhere shapers calculated that there shall be certain length of completely flat section with then certain length of progressive rocker to make the board plain fast and at the same time not catch water with too steep abrupt rocker at speed..?

Calculation.......
Feel spread...22" plus adjustment for weight.
Spread front strap to mast base.
Now, how much more float to uphaul and tack without getting ridiculous in nose thichness.

Not sure I got it but thanks anyway.