LD Max, I went through the same evolution a few years back. All I found in documentary form was in PANS-OPS, as follows:
“A serviceable altimeter will indicate the height
of the altimeter in relation to the QFE reference point
within a tolerance of +/- 20m or 60 ft for altimeters
with a test range of 0-30K ft”
PANS-OPS Vol 1 Pt VI, Ch2 para 2.2.1.3."
I looked in JARs but discovered that the limits were in JTSOs (Joint Technical Standards Orders), but that JTSOs hadn't actually been written yet! So I felt free to rewrite our SOP according to ICAO.
There are other standards in there for altimeters with a greater range also.