I use the AST tool to and am very pleased with it. It's a little tight to get on the rear belt, but once you've done it it's clear enough to repeat.
It's important to use button 4. The gauge that comes with the tool will indicate button 5 but that doesn't give the right tension.
Don't know why, but I've "calibrated" it against a Ducati tool, and the manual measuring belt deflection method and button 4 is the one to use.
I've used it a fair few times now
and it is staying in calibration very well.
I don't think you can go wrong for the money IMO.