Building a new 170 x 50 ultra light wind twin tip. Rocker is continous around 1.25 inches. I'm thinking around 4-5mm concave. Any thoughts on how far to run the concave? Just between the feet, to the fins or full concave all the way from tip to tip? Pro and con for light wind start and upwind angle. Also what do you think about no concave?

With 1.25 inches of rocker I think you gan go a bit deeper with the concave.

You basicly want the concave to take rocker out of the centerline of the board, bit without going right to the tips. Avoiding the tips will improve water breakaway.... the way the water leaves the back end of the board.

Leave 1 to 1.5 inches along each rail un effected by the concave.

For my new table I'm just using fiber board as the surface so I can screw the table surface right onto the frame allowing me to flaten out concave at the rail. Just cover the screws with packing tape and wax it down with mold release.

I would love to hear a decent explanation of why a concave bottom should flatten out at the tips.

Every board I have tried that used that design felt really slow and draggy.

The only explanation i have heard which sort of made sense said this design improved control by adding drag - is this really what you want on a lightwind specific board?

The main effect of concaves is to reduce rocker and thus increase a little bit speed and acceleration of the board. I have made identical board with and without rocker and tried them out, since than I only make flat bottomboards and don't bother with the hastle....its not worth the extra effort.

Concave adds some stiffnes to the board and therefore progressive flex through the board. Not having concave to the end of the board, leaves the tip flexier.
If you make the concave too small, let's say just between the feet, it is going to produce vacuum under the board, therefore you will need a lot of strenght to get it out of the water when jumping. I guess it produces more drag too.