To be honest, I think you have all the information you need. Your boards are never going to experience the full range of potential movement. As profchris has suggested, allowing approximately 2 - 3 mm per board for shrinkage and expansion should cover it.
But if you really want to dig around and explore a bit further into how wood reacts to water as it's adsorbed and desorbed, you might look here. It's only a start into the fascinating world of timber technology (well, fascinating to me anyway), and perhaps not directly relevant to what you're doing now, but you may find it of interest. Slainte.
yes think will go with 2mm.
will take a look at that link thanks, it’s a world I have only just started looking into!