Another trick is to time up using std cams, then loosen a cam cap, lay a piece of leather on top and tighten down, then mark the cam by the cap so you can see it hasn't moved, replace the std pulleys with the verniers, bit long winded but works 100%.
When you come to fit the verniers just leave the adjustment quite loose so they set/rotate as you tension the belt BEFORE tightening the pulley adjustment bolts.
tabetha