Thanks Greywolf that's the formula is was looking for - since I will always ride solo and never with fully loaded panniers I figured the 30mm drop should have been safe enough ? but for some reason theses one are only giving me just above 1/2" when advertised at 30mm
Ah well back to the drawing board
BTW anyone doing this the job it is simple took me all of 10 mins on my own ( with centrestand )