Hi. It looks like 115 litres tank is in manual just in case but has never been implemented in module. On your other question: Yes, A-8`s fuel system is not straightforward even after reading manual....but I figured it out. What we want is to deplete external tank as first, right? that would not happen under default levers and circuits settings.
You need to do this:
1. Select FRONT FUEL TANK CLOSED selector lever [second position from top out of 4 ]
2. Keep rear tank pump ON
3. Turn OFF front fuel tank pump
4. Turn ON external tank pump.
5. switch fuel gauge needle for rear tank
6 Take off and fly
7. Once fuel in rear tank will drop from 290L to 240L fuel from external will start to feed it so needle will stay on 240L for long time before the rest of the tank will get depleted. Then turn OFF ext. tank fuel pump and drop the tank if you like...but you don`t have to.
8. Once rear fuel tank is almost depleted, switch front tank pump ON and keep rear pump ON as well. Move fuel selector leaver to topmost position.
9 Now the rest of rear tank will keep getting depleted while front pump will keep pumping from front tank, too.
10. Switch fuel gauge needle for front tank
11. You will keep running on front tank only. You can turn OFF rear tank circuit breaker if you want.
I was puzzled as well by fuel transfer system...so dived into it in manual, tested in DCS and found that this approach works. Good Luck!