You can use several programs to check the optimisation of the optical components of a scope(eg. secondary size, top ring inner diameter, etc). "Newt" is a common one but I used a speadsheet by Andy Martyn instead. Also does focuser and behind secondary baffle calculations. Just enter in your own values and it will do the job. Email me if you want a copy as it won't attach here.
For my 10 inch I used a single thickness of 12mm Gaboon Ply. For the 22" I used 2 layers to get 24 mm. Stiffness depends on design too. On both these scopes the 3-vaned spiders are bolted to where the poles attach to the top ring. The main potential for flexure is then the focuser board. I would go with 15mm then reinforce around the focuser if necessary.
50mm between the inner and outer rim sounds about right - also need to leave room for the thick poles/clamps if going with a Highe design.
Poking around Highe's and Dobstuff's websites should give you plenty of food for thought.
