Refer to the photo in this post
http://www.cloudynights.com/ubbthrea...2/vc/1#3906061
The 10mm estimate is the distance between reflections P & 2 which is an accurate estimation. The triangle spot perforation diameter is known to be 6mm. If you reference it, then you can estimate the distance between reflections P & 2 centers to be around 10mm.
The 8X is not because of 8 passes. In fact, Reflection 2 is only 5 passes. The 8X math is kind of complicated but it is a true statement.
Q 1. Is that a severe enough error for an autocollimator to negate the benefits?
Yes, because an average quality laser collimator can do better.
Q 2. Wouldn't most lasers and or laser+focuser give errors of at least that?
Yes. That is why autocollimators better be of superb quality to improve collimation above and beyond what other tools are capable of delivering.
Jason