Also what on earth are "600 series" flanges?? or "Skillet blind" for that matter. Do you just mean a blind flange or a spade?
Could be anything, but if you mean class 600 flanges to ASME B 16.5 then yes, use ASME B 16.48 as noted above.
But please be a bit more precise - that's what design engineers do.
If you're trying to use some home made flat plate then you need to do a design analysis to ASME VIII or similar. Note at 100 psi this plate has a force of 5 tonnes being exerted on it so no 3mm thick piece of "steel" please....