A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robin Stuart
Date: 2019 Mar 18, 09:17 -0700
The 20NM limit in the NA is probably a general rule of thumb and assumes that the observer typically avoids taking sights when the object's altitude is very high and hence the GP is quite close.
I'm not sure whether this directly answers your question about published guidelines but the only other place that I recall seeing LoP curvature adjustments is in Pub 229 Section C.1 Adjustment of Straight Line of Position.