Nigel, sorry for chiming in late but I was on holiday!
I'm still pretty new at all this so best to double check any info I give. If you are making a sliding box camera similar to mine then there are two factors to determine how much "bellows" you need. Firstly the focal length of the lens and secondly the type of photo you'll be taking.
So, as an example, if you were going to be using a 90mm lens and mostly taking landscapes you would be most interested in the infinity focus end of the bellows which would be 90mm bellows extension.
At the other end of the scale, lets say you wanted to take flower photographs with a 210mm lens so that the image on film is the same size as in reality ie. if the flower is 10cm across in reality and you want it 10cm across on film you need a bellows extension of twice the focal length, in this case 420mm.
So, for portraits, again you'll need to know what focal length lens you're using and whether you'll be doing full length portraits or head shots. After that its a bit of experimenting.
The good thing about foam board is the it's relatively cheap and it's not a big deal to trash it and start again.