Yeah, and VOB has a lathe. I thought he might have shortened it and cut new threads. Now that I think about it, it doesn’t really make sense for him to cut down the double length tube.
5 meter is bit short, you may be off a few percent, some people even claim that 20 meter is not enough for good collimation and thus correct measurement. And technically they are correct.
But then I claim that the error of their luxmeters is more significant than the difference of measuring between 5m and 40m.
Moreover, the focuspoint of not all GT’s is at infinity as it should be, so if you have a very unlucky GT, the best collimation could even be at 5m.