Mark.
The venting of the MBT 3, 2 and 4 port and stb is arranged as seen on the sketch below. 2 and 4 port and stb are opened simultaneously. MBT3 port and stb likewise.Thus you may say they are operated as one tank, however each tank venting can be shut off by a valve next to the tank in case of a possible damage to the venting duct. I believe the flood area indicated on plate 28 is the total. If you calculate the tank volumes in relation to the floodareas you get for the MBT 3 total volume 47,7 m3/ floodarea apr. 2,76 m2 = 17,28. For the saddle ballasttanks 2 and 4 total volume 49,6 m3/ floodarea 3,167m2= 15,66. Be aware the MBT 2 port and stb has a venting in the aft end for residual venting. When diving or surfacing at an angle you shall have difference in pressure in the ballasttanks as they are at different depth.
Tore