Kim you cant go by that at all... on my wintec saddles for long griths Champ uses different girth lengths then on the prestige... its impossible to say without seeing the horse and the saddle..
your a a livery yard, surely there must be several people that can loan you a girth for a try and then you know what size you need... I have several 110cm so you can loan one of them of me if you want.