Thanks for the tip, but I've already got some data. First I started as Venice in 1419 (AGCEEP), where they control Dalmatia from the start. It turns out that the WS-cost is BTV+4 if none or minimal fort, BTV+6 if small or medium, BTV+8 if large or mighty, and BTV+10 if maximum fort. Adding a manufactory (and a shipyard) didn't affect the WS-cost. I hope someone would like to validate it.

Be aware that if the BTV is an odd number, the warscore will drop down to closest even number. For example, BTV is 9 and fort is minimal (+4) -> WS-cost is 12.
I also tried to capture/lose provinces with CoTs, but haven't found a connection yet:
Zacatecas: BTV = 8, minimal fort, WS-cost 32
Alexandria: BTV = 9, small fort, WS-cost 34
Flandern: BTV = 17, small fort, WS-cost 42
Thrace: BTV = 18, small fort WS-cost 46 (it's WS*2)
Someone sees the connection? Edit: Looks like its WS-cost + 20, but the last one doesn't fit...