The method I used to use for checking belt tension is to set it so that there is 3mm of belt deflection between the cam pulleys.
Wind the tension up tight, scribe a line across the top of the belt on the rear belt cover, then back the tension off until you get 3mm of deflection using "moderate" finger pressure.
It's worth tensioning the belts tight, turning the engine a couple of turns to settle the belts, then setting the correct tension to.
Always set the tension going down from tight. If you go too loose, then tighten them up and go down again.
The tension pulley should be turned anti clockwise to apply tension.
Depending on the tensioner you use, then use the above method as a check/calibration.
I use a Snap-on unit now and use button 4. The gauge suggests using 5 but that sets it too loose.