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Worked out the effect of SAEP for a specific example, my 20mm 100° explore scientific eyepiece, with rated 15mm eye relief. But I wasn't able to simplify the maths into something presentable.
If there was no SAEP, and eye pupil is 6mm:
Eye position is 12.5-17.5mm vertically, and +/-3mm horizontally.
With SAEP and eye pupil 6mm:
Eye position is 11-16mm vertically (effective eye relief is 13.5mm), and +/-2.5mm horizontally.
Now if you try to use this eyepiece in daytime with eye pupil of 2mm, assuming the exit pupil is also 2mm:
Eye position is 13-14.5mm vertically, and +/-0.5mm horizontally! Outside this cubic millimeter of space you will get see partial blackout and kidneybeaning.
In extension to the first post, here's a diagram that shows how the sweet spot of eye placement works.
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