Hi again
That tensioning tool is Fiat 1860745100/300. I found an equivalent made by XXL in Germany, see XXL-Automotive.de Their part number is XXL-117652 and the pdf instructions (in German) are attached. Price 103.75 Euros plus delivery, so not cheap unless you plan to set up a belt changing business.
From what I can gather, your belt setup uses a fixed tensioner rather than the more modern spring loaded type, so setup is perhaps more critical. I suspect if you took this job to a non franchised garage they would simply press the belt with an oily thumb and use their experience to set the tension ! Belt tension can be measured with various tools, but unfortunately I have no spec for what it should be, I would guess at about 500 Newtons (51 kg-force).
The last possibility is that a properly tensioned belt should vibrate like a bass guitar string when you pluck the longest length. A frequency of 100 Hz would be typical, but this depends on the tension, the length and the weight of the belt.