This is the right formula to avoid disappointment 'do not expect too much' because as you say bonuses and calculations have always been a mystery ~
My wagering amount is around $50k since the last monthly bonus time, and I'm down like a few hundred dollars. Actually I'm expecting something between $70 to $90 from the November monthly bonus. It will be disappointing if I receive lower than $70.
All I know is my monthly should definitely be lower than last month, but it might still be in the very low 3 digit area. Let's see.
You are still hoping for a 3 digit bonus while you have said your wager is small in this month. I'm wondering how much wager you made since last monthly bonus time! Most of my wager on in-house games. That's why I'm not expecting a bigger bonus from the monthly.