NavList:

For me, the position of the sextant arm within the box is all about how we hold the sextant securely and safely when removing and returning it to the box.  Because the sextant handle goes into the box first, we must hold the sextant by the frame.  In doing so we want to avoid contact with the mirrors and any force on the sextant arm.  In terms of distance between the hand and the mirrors, the ablity of the fingers to hold the frame securely either side of the arm, and balance (which is important in a seaway) this suggests an arm position of about 40 degrees (plus or minus).  This is about where most boxes place a marine sextant arm.

You mention the Ebbco.  I bought an Ebbco Special new many years ago as the backup to my primary sextant (satellite-based navigation systems were available to civilians).  Yes - its box forces the sextant arm into a narrow range, but that range is perfect for a handhold into the x-shaped slots in the plastic frame.  And that is why it was designed that way (I believe).

The Freiberger has a different frame design but I believe the same theory holds true.  I have never used the Freiberger and note from photos that there appears to be more freedom of position of the arm within the box.  I would still choose the position which allows me to grasp the frame securely as I describe above - whatever position on the index arm that may be.

My routine at sea is always (1) a safe hold on the frame and remove the sextant from the box, (2) with the other hand, take a secure hold on the handle, (3) place the lanyard over the head and pre-position shades at their likely best combination and only then (4) go on deck.  Returning the sextant to the box is just the reverse.

By the way (off topic), last year over the space of three days, I took some sun sights with the Ebbco for fun, together with my Tamiya and my original Heath for comparison.  It was the first time I had used the Ebbco in years but it was a matter of minutes to tune out the errors to within an acceptable range.  Both mirrors could benefit from re-silvering but were adequate.  These comprised morning, merdian and afternoon sights.  Over the three days the Heath (the oldest of the three) performed best, followed by the Ebbco and third was the newest (the Tamiya).  All three were within a narrow range which means that the differences were irrelevant and down to the user - especially when solo timekeeping.  All position lines ran agreeably close to the GPS position and sun sights run forward (from a stationary sighting position) gave excellent "fixes".  This gave me confidence that my 50-year-old plastic sextant, always well maintained, still has a place on board as an emergency backup.

I am curious why you prefer to box the sextant with the arm in the zero position.