Yup I have one of these too and put them to use today - Used paragon tube blocks as references and seems like I got a good enough measurement.
Thing is in the end its just hole saws - and one of my hole saws developed a jiggle half way through - so the cut ended up being a bit off anyways 
What were you using for a holesaw arbor ? The Paragon-style ones are are best.
Here is what I do for main tubes, I find it fast and accurate;
I clamp two PMW tube-blocks to both sides of the tube, on a good surface plate, 15-20mms from each miter to be cut. (I predefine the spacing between them on my design though, and try to clamp them as accurately as I can distance-wise, this is critical) So you basically have two datum planes close to both (or three if it’s DT) miters, seperated by you predefined distance.
Now you have top/down and sides of the tubes defined by the tube-blocks. I scribe all the centerlines at top/down/sides (by a height gauge on surface plate)
Then I measure on the design the distance between each block and its neighbouring miter, then scribe a mark at this distance from the tube block, with a sharpie, to the relevant centerline on the actual tube, to define the start of cut.
I clamp the tube by the tube block, at the required angle, to the millling machine. I attach a sharp point centerfinder to the mill chuck. Align the mill center to the centerline on the tube (by eye). Then switch to holesaw, and approach bit by bit towards the sharpie line, touching the tube, until the cut mark is exactly on the sharpie line. (Let me note; you can try to find the center of tube whole day, to within less than 0.1mm accuracy all you like. Yet my experience is that; there will be usually an amount of flex in the system, and the holesaw will push the tube towards the rotation direction more than this amount anyway)
This produces very good results for me; if you’re careful with the measurements, and it’s not so hard to take robust measurements with this method. Usually the only thing I do with the file is to smooth out the burrs.
I used to set the tube around .003" off-center to accomodate the tube deflection from the holesaw.
Both “ears” were then even - unlike when I had the tube dialed in to Center.
Exactly @LemmyHerse . That’s what I do sometimes too. But it’s a bit tricky; the offset value depends on the flexibility of the tube. The smaller the tube diameter and the further it extends from the tube holder, the more offset I need, as I found out.