Look at the lenght of your old bolts, add booster width. Eye-ball booster against mount, wanting it to clear by 1mm or so, just add 3mm for the recese on mount and thats your spacer length. So spacer plus booster plus old bolt = new bolt. 
Umm does that help? 
P.s most boosters come with 16mm spacers and 45-46mm bolts, but I use just 12mm to keep it all compact. :blink: