well, I get what you're saying, I'm just thinking that it's possible that by setting the yellow to 60 you effectively change the size of the storage considered for the alerts - 60 becomes the high end meaning 40 is 66.667% of the full tank - in this case.
whether this is the intended behavior or not, is up to the devs to convey, but I was iterating that moving the yellow marker ('output to belt/pipes' threshold) higher would change the conceived size of the tank respectively - 80 would make 40 be 50% full and 120 would make 40 be 33% full, etc. etc.
I'm not saying I'm right, but why don't you test it and see if that's right or not and build your alert system accordingly ? and if I'm wrong - let the world know - continue the quest to change this behavior, if that's what you think it should be - far be it from me to deny anyone what they want (especially, as I'm not part of the development team, I'm just a player of the game, trying to help a fellow engineer understand how it works)
(not saying I have the answer)