NavList:

Good day Norman, thanks for the heads-up on quadratic equations being roped-in for the job.

I am speaking purely of myself, I have always found much improved measurements taken when in a seated position and supported with aid of chair arms, or even a length of wood placed twix chair arms. This also gives a good elbow support

For low alt's , I put the AH  chest high, on a tripod.......obviously , location sometimes makes options very limited...my most approximate-challenging circumstance, has been from a free standing position and the AH on the gound.

For unavoidable situations like that,  I use a deeper than usual  fluid recepticle, sometimes having to resort to slim wedges too.

Lawrie