Paul
For the superseal there is a cradle spec in the handbook.
The forward support is positioned just in front of the keel and the rear support 2.16m overall supports aft of this. The item gives details of the hull shape for the bearers. I would expect the 275 to be similar.
The "table support frame" I made for rascal follows theses dimensions, except that I used flat timbers (faced with offcuts of vinyl flooring) to sit on and then made shaped timber wedges which I tap into place each side on each bearer. If the hull is not dead center on the cradle is does not matter, the wedges always fit.
The higher the hull is supported off the ground the easier for hull maintenance, mine's about 700mm. I always lower the keel on to a block for the winter.
Philip