HAL,
As a christian you pay 25d for each city province with the state religion, 50 for other Christian and 100 for non-christian. Since you are protestant IIRC, you have to add a further 3d per province. Also 5d per colony and 2d per TP.
Add all and you have your base stability cost.
Now multiply because of DP sliders (Serfdom and Innovative).
Then multiply because of BB. That was the one I suggested ingame was the reason your stab cost suddently increased. The formula is quite complex but basically you should try to never go above 50% of your BB cap.
Then mulitply because of vassals and FAA's.
Then mulitply with inflation.
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Whenever your stab costs suddenly increases with a lot it is because
1. You suddenly acquired a large number of provinces, relative your number of provinces before, especially if they are of a very wrong religion
and/or
2. Your BB points just went over either 50% of BB point cap or reached 100% of this cap
I could see ingame that you were over the 50% mark. When you took Bangkok you perhaps DOWed without CB (4 BB points) and then took it in an offensive war (2BB points) and then lost it in a fake war with AUS (-1BB point). That is in total +5 BB points for you. Huh, costsome HAL

o
I can see in the saves that you had 33BB when we started the session and 29 when we ended it. Your cap cannot be seen at the Russian site. Load the save and check.
And remember that the BB cap depends on the DIP skill of the monarch (that also can be changed by events).
Assume you have a good DIP monarch and an excellent minister event working, taking you down to below 50%. Then you take Bangkok, gaining 6BB points, and the event stops working, perhaps your good king dies as well, now you look at your stab cost and you see: shit 9000d. What happened you ask! Well, it was the 50% level of the BB cap that hit you. The difference in size (+50%) is eminently possible, not least because the multiplier comes ahead of the inflation multiplier in the formula.
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It is paramount in EU to "always" lie below 50% of the BB cap. Whenever I see one not doing it I laugh silently for myself. When, as happened in this game, such a person even spams his opponents with pirates (thereby risks getting extra BB points) I open a bottle of Champagne and celebrate!
