So this is only a gcode file for prusa printers, but a good guage if you can print it.
https://www.prusaprinters.org/prints/46639-tension-meter-for-the-gt2-belts-of-i3-mk3s-or-prus
Also for petg only. If anyone can find the stl for it feel free to add it here.
I printed it on my prusa mini, and use it here and there on ny other printers when i feel curious or suspect issues. Though its a bit tricky on my x axis belts XD.
I quite like the idea. The only problem I see with it is that they don’t specify the brand of PETG to be used. The design obviously uses the flexible property of PETG to cause it to bend an anticipated amount to be measured on the scale, but what if the brand of PETG used is more or less flexible? I mean, to be fair, the exact belt tension isn’t that critical (as far as I know) so I don’t suspect it will matter if your off by 20% by using a different brand of PETG, but still. It would probably have been helpful… Then again, I must confess to not having read the entire page, so maybe it’s on there, somehwhere.
Good point, i didnt really think of that, though id make a lazy assumption of 2 things. 1 you have a prusa, and 2 they would most likley recommend prusament/the gcode would be set for prusament.
This assumption comes as its from prusa themselves.
Hmmm, maybe ill give it a try sometime with different brands to see if i get a different result.
Heres the youtube link to where i first heard about it.
Fyi its only 20 secconds.
Well i know what i am printing this evening.
I want it… I hate the way I test my belt tension.
‘Cries in Creality’ 
I use this all the time on my MK3S. I recollect in the article that is was specifically designed for Prusament PETG which I printed mine with. Printing with any other material such as PLA is different than PETG and the guage may produce different results. https://omnexus.specialchem.com/polymer-properties/properties/stiffness I made a test jig and tested other PETG brands. The PETG samples were very similar. IMO just print this guage with any brand of PETG, it will be close enough. Check the belt tension after setting with the guage and use your judgment.
belts are fairly universal it should work fine if there is the room.
For sure! It looks like if I had the space I could just snap it onto one of the belts.
Sadly on my current machines the belts run through the aluminum extrusions. Just no room to fit it in I think.