Thanks for the reply chris,
The rear deck section is aprox. 8ft x 7ft (2.4m x 2.1m), Normal foot traffic, coolers etc.
I just wasn't sure how thick the needed to be over pvc foam as I never used it before, didn't know how much flex would it have. My estimate was based on over laying plywood. 3mm isn't too bad, needing that thickness I might use some 1708 or roving I have instead and make it slightly thicker. I don't like using CSM, too messy.