mat,

basically it is trial and error dude,

but to give you an idea on the left below is a totally std escos and mine on the right

they bothing running same size rims and tyres




ok now below is mine on same suspension height, but on 18" rims with 235/40/18 tyres

as i know you have 18's that will give you a rough idea of the height you need to reach, or just a bit higher in your case.

hope it helps






CLICK to enlarge both pics