What works for me is I use Allen keys as my tool for measuring. Using say a 3mm gap I just barely put any presure against the belt with the tensioner when I install it. Tighten the bolts all the way. Then once I pull the stopper bolt it opens to about 4.5mm. Rotate repeatedly and fine. If it changes its usually is maybe a mill to tight or lose and then I just apply a pinch more pressure on install or less. Don't touch no piston.
"torque gets you off the line...& hp gets you over the line." - TTKeeper