Thanks for the reply, your advice is on point!
If you are planning to build more than one board, I would suggest, you build yourself a rocker table. Although it is definitely not necessary, it helps a lot. MIne is 2,50 long allowing for boards up to that length.
The good thing about the rocker table is that it sets the rocker throughout the entire build, which is extremly helpful, if you press deck and bottom onto the ribs, the abouve mentioned tendeny, that the board changes its rocker due to internal wood tensions is diminished.
A rocker table is nothing else than a woodden frame with threaded rods on each side. A nut goes onto each rod and another vertical piece of wood combines the rods on each side of the frame. I added a plank of thin flexible plywood, which is covered with some blue plastic to avoid that something stucks onto during gluing and added a secondary vertical bar, to be able to press the things down.
Get your rocker defining stringer and adjust the bottom matching the rocker and use a water level to balance the nuts on both sides.
This is my last and by now best build board. I used a goretex vent and it works beautifully...