I refer to CPL final test of the CPL performance textbook Q33. Given that I am asked to load fuel into the auxillary tank, I instinctively assume this to be a aft limit problem, hence I used the flow chart. This is because I thought that adding fuel to the auxillary tank makes the CG more aft (vs loading in main tank. However, the answer I got was out of proportion when I used this method, and checking the textbook answer, I realised that this is a foward limit problem that requires the use of the chart. My question is thus how I can rectify this mistake for the future, and recognise that this is a forward limit problem rather than an aft one, when encountering these types of questions?

The best way to deal with this problem is to make a quick rough check on the envelope (just with your fingertip will do). Even if you don't use the envelope to solve the problem it will help you to see which limit you are close to.

Then consider what action is to be taken and which direction that action will cause the centre of gravity to move.

If it's the aft limit, that's great because you can always use the formula/flow chart. If it's the forward limit, you can only use that formula/flow chart if you are shifting weight (weight remaining constant).

In the case of the auxiliary tanks, adding weight will move the centre of gravity aft, but the forward limit will also move aft with increased weight, so you can still end up out of the envelope by adding fuel to the auxiliary tanks.