I have been using one for years...

Regarding the setting of the diamond wires.... I got recommended benchmarks from the person that made my mainsail... They gave me a few settings depending on the conditions. I just took the middle setting and went from there...

As for them being accurate, all I can say is that I saw a Loos gauge in a aircraft mechanics tool box that he used for light aircraft rigging. I would say they are pretty much "close enough for government work." Note, this is all they do; that is build tools for cable/wire rope industry. I think they have it figured out.

You don't have to buy one... I always lend mine out to fellow sailors and collaborate on settings etc...

Also, if I could recommend, you may want to upgrade your diamond wires... The Inter 18 and Nacra F-18 used 1/8" here in the US and the recommended tensions/setting from my sail maker was pushing the limits of the cable... Loos recommended tension at no more than 25% of rated breaking strength. I upgraded mine to a 5/32 and was able to "honk it down" with plenty of room before it maxed out..

Here is some info from their website...

http://loosnaples.com/how-to-use-pt-series-tension-gauges