Your foam laminate will take whatever shape you put in the rocker table. For this reason you really need to pull the concave into the surface of the table. I've done this on my last two boards by jamming a space inbetween the underside of the table surface and the table's frame so that it pushes up in the middle. Its not particularly scientific but you can control the amount of concave reasonably well.
I'm trying to work out a way of putting double concave into the boards with a rocker table - not so easy!!!