NavList:

There are quite a number of possible variables for your mild inaccuracies, but I would wait for the new scope before going to great effort tracking down other error sources.

I have the 7x scope on the Astra, which I've been using regularly for years in a wide variety of underway conditions. At the same time, I have used lots of student sextants (Mostly Astra's with the basic scope.) Comparing my results with the two different tools on any given day, the difference in clarity and precision of the bigger scope is pretty impressive. 

In the meantime, as Frank has indicated, your results would certainly get you where you're going in most cases.