Hi Kiwibraider, core diameter times 1.5 divided by number of strings you wish to use is what I use. Your string thickness times four will tell you how much your braid will add to your core diameter to figure the size core you want. For example, I generally use 1/16 thick strings for hackamores, so my cores need to be 1/4 in. smaller than what I want for a finished hackamore. I use rawhide, so roo may vary, but kind of a rule of thumb. Hope this helps, you can make the metric conversions lol.