O.K. so my plan to get the basic proportions looking right is to work with the 1,168 golden number or whatever folk call it,
Im keeping the original tank so all my other sizes come from that ( Im working in inches here) tank size is 28 inch / 1.168=23.97 to give me the overall seat length, Tank height is 9 inch /1.168= 7.70 to give the seat hump height ( i hope this is making some sort of sense cos im rubbish at sums)
maybe the pics will explain it better....