NavList:

In my small collection of sextants, I too have one of these Davis Mk3 'life boat' sextants".  And sure, they have their problems not the least is plastic thread wearing of adjustment screws.  But, you can take surprisingly  good sights with them if you are careful about first determining their index error and taking care to read the arc angle vernier correctly. 

The best method for determining index error uses the sun and takes two shots. In the first shot, the reflected view is atop and tangent to the direct view image. In the second shot, it's the reverse with the direct view on top of the reflected. With the sextant's trad horizon mirror, you need to make these two shots and recorded angles with care.

The proceedure is described in this NavList posting   http://fer3.com/arc/imgx/solar_ic.pdf 

Try it, it works very well.

As far as side error goes, a small amount can be ignored but you can not ignore the index error.