I've created a short video:
It's very much at the amateur end of the spectrum, but I hope it helps a few people.
The tension should be 20-30kg.
This is the device:
http://www.amazon.co.uk/gates-7401-00071-Krikit-I-Gauge/dp/B000MUTAGSThere's also an app, which I haven't tried yet:
Carbon Drive Bicycle Calculator by Gates Corporation
https://appsto.re/gb/2lkiA.iI'm not certain this is the correct one, but I'll endeavor to give it a go and report back.
Your Zero dealer can supply you with the Krikit. I paid a little under £20 for mine but you may be able to get it cheaper elsewhere.
I'd like to see Zero supply one of these with each bike as I have a feeling that many people may not get round to buying one themselves.
It's very important that the sprockets are aligned perfectly. I had a belt break on me recently and I/we suspect misalignment was the cause. I managed to recover the belt and send it back to my dealer who will send it on to Zero for analysis. If I get any feedback, I'll let you know.
Also, try and avoid getting debris, like sticks and small stones caught between the belt and the sprocket. This can cause over-tension and may cause problems. That might have been a factor in my case, but it's hard to say as the belt failed many days after I took the bike off-road. I also cleared the debris before riding any distance.