I'm not an easy sell for rules-of-thumb, left unqualified they can be conditionally true or an oversimplification of a concept lost or watered down. Anyway let me run this one up the flag pole.

*** HGT = 126 + ambient ***

(1) This a good one?
(2) Can you also say HGT = 126 + RA DB for a Heat Pump in Heat Mode?
(3) I've read of charging by HGT (scary), but even if true it would have to be under some base line, induced ARI, or peek demand conditions. I'm not willing to set or adjust change by HGT alone and then walk w/o looking and SH and SC to complete the picture.
(4) Most importantly, can HGT = 126F + {ambient/RA}DB be used as secondary information to confirm other indications of charge or heat exchange problems?
(5)Anybody know the basis for this rule-o-thumb? Lift? CR? Specific volume? PFM??