[Q]

That's how far I understand it:
Accept Treaty Base Anger Level := 30
Accept Treaty Anger Modifier Per Higher Treaty Level := -5
Accept Treaty Minimum Anger Chance := 5
Accept Treaty Anger Modifier Per Other Wars := 30
Accept Treaty Minimum Time From Last Treaty := 10
Accept Treaty First 50 Turns Modifier := 10
Accept Treaty Anger Modifier For Percent Stronger Player := 150
Accept Treaty Anger Modifier For Percent Stronger Player Amount := 50
Accept Treaty Anger Modifier For Percent Weaker Player := 50
Accept Treaty Anger Modifier For Percent Weaker Player Amount := -30
You have a base anger level of 30. If the AI has a higher anger level against the race that is proposing the treaty, it will reject the proposal. Now if the empire is much stronger or weaker than the AI this level will be changed by the amount +50 (stronger) or -30 (weaker). This means the AI will rather accept a treaty from a stronger empire than from a weak one. What is "strong" or "weak"? Strong is in our example an empire with 150% or more score of the AI and weak is a score of 50% or less. So this would make the AI accept a treaty from a empire with more than 1.5 the AI score even if the anger level was up to 80.
As you see there are other modifiers for the anger level as wars with other empires or the beginning of the game(makes the AI accept a treaty at higher anger level). And higher treaty levels like military alliance and partnership are only accepted at lower anger level. I am not absolutely sure what the "base treaty" is but I think it is the trade alliance. I do not know what the Accept Treaty Minimum Anger Change does. (I would expect a Maximum Anger Change Level, if the anger has risen in the Last turn above a certain value).
You can create very different behaviour of the AI by modifying these numbers, which is really one of the most beautiful features of SE IV in my opinion.