Fully charged static, the 31 Lifeline that owns me, will drop from 12.79 to 12.77 with a 3-amp Bi-PAP load. With an LED 30-watt load added it drops to 12.66
What you are doing is invaluable. Later on, when a question of capacity arises the trends and tendencies you record today can be compared to suspicious activity later on. If in fact the slump has noticeably increased do a conditioning charge followed by a capacity test.
NOTE: As a general trend the more an AGM plays the part of a high CCA starting battery the less slump a person shall see. I love the idea of a heavy AGM with less than stellar CCA but with superior capacity.