[Q]

This seems to be rather unknown territory.
Therefore I made a little test myself:

Two empires under human control except the politics minister. The corresponding AI anger.txt file were modified with all events having an effect = 0 except the "regular decrease". There empire 1 had a value of -50, empire 2 a value of +50. When the empires established contact the moods changed rapidly for empire 1 to brotherly and for empire 2 to murderous. I completed a few more turns just to be sure to get the extreme values of the moods.
Then I changed the regular decrease values to +10 for empire 1 and to -10 for empire 2. This resulted in a clear but slower change of the moods towards the other extreme. From this test I believe the correlation between the AI mood/anger value and the displayed moods is the following:

0-9: Brotherly
10-19: Amiable
20-29: Receptive
30-39: Warm
40-59: Moderate
60-69: Cool
70-79: Displeased
80-89: Angry
90-100: Murderous

It seems impossible to get values over 100 or negative values.
My personal conclusion is that you should choose the values in the AI anger.txt file rather low otherwise you get extreme changes of the mood very rapidly. And there is no use to have in the AI politics.txt file conditions for anger below 0 or above 100.