[eligunn]

My experience is that you can totally tension the belt just by using your own judgment.
I have personally seen reputable Porsche mechanics reach in there and give the thing a tug or two and then tighten the nut and let her rip.

I have found that there is a visual reference you can use to know whether or not it's in the ballpark.

First of all you want to turn the tensioner counter clockwise to tension the belt. So that it gets closer to the water pump pulley.

At this time the point of the hex part of the tensioner you have your wrench on points right about the center pf the water pump pulley, and you can make minute variations from there. If you are pointed at the center of the pulley you should be close with the tension.

I think the tensioner tool just complicates things and makes you question your judgment. However, the consequences are severe if you screw this up. The last time I had a head rebuilt it was about 300 bucks just for the machine shop bill plus a head gasket on top of that.