I have only seen one of the Tristock stringers on an ARF. It took be a bit by susprise, but it made sense. I thought that they were 45-45-90 angles with the 90 down and flat side out. Given the way they build though it would make sense for the stock to be 60-60-60 as which ever side they put down in the grove would be correct. I don't remember which guy at the field had that plane, so I can't check. Do a bit of simple trig. Measure each side. With that info, there is only one answer to the angles.

Don