It's kind of a black art
You mean "Tight, but not to tight..." isn't a good enough explanation
I've made this observation before on the forums, but it's funny to watch Mikado pilots from a distance at events. Sooner or later, they all casually walk by someone else's Logo, and sneak a check of the heli's belt tightness. It's like no one knows for sure if their belt tension is good, and they all want to see how theirs compares to everyone else's
Been doing the Logo thing for awhile now, and my opinion is, if in doubt, tighter is better than too loose. Almost nothing bad happens if the belt is too tight...and I can't think of or heard of a crash ever caused by a belt being too tight. But bad things can happen with a belt too loose, and crashes have occurred as a result.
Here goes a probably lame attempt to offer a guideline. I'm not using actual sizes here, but let's assume the belt is 5mm wide.
So, when you squeeze the belt together forward of the rear pulley just as it goes into the boom, you should not easily be able to squeeze the belt together any more than it's width. Which in the example given above, a properly adjusted belt would mean the distance between the top and bottom of the belt when squeezed, should be no closer together than, and would be about, 5mm. If you can easily squeeze the belt together closer than it's width (or worse yet that it touches itself), then it's too loose. If you really have to squeeze hard to get it together about its width, then it's too tight.
Of course, now how to you define the terms I used above like, "...not easily be able to squeeze..", or "...really have to squeeze..."
Like I said...it's a black art (-:
Hope this helps at least a little.