This is about the Heresy competition right? Andy explains a bit more about it
here. You have to keep in mind a possible 10-20% size loss in the direction perpendicular to the mould plane.
To help you understand this:
Here you see some details on moulds are made. Moulds are compressed and heated when vulcanising them and are compresed again when they are used for casting. Both of these actions can create a size difference from the master.
If I remember right, after a mould is taken from the vulcanizer and the masters are removed the rubber will still settle itself somewhat which can cause distortion. Also, the pressure of clamping moulds together for casting can cause this and this become worse the warmer the rubber becomes (it also becomes softer). Most distortion will appear in the direction in line with the pressure, so perpendicular to the mould face.