I tried to do some stuff with circuit network and found out that tanks are full when having 24999 units of fluid in them.
Im not sure if this is or isn't intended but it seems to me it should be simply 25k.
[0.15.11] Tank capacity is 24,9k
[0.15.11] Tank capacity is 24,9k
Hm.... so we have a mystery donor... intriguing.