Math Major?

I am self-studying Further Maths (Edexcel IAL) and so far I am doing quite well.

I've always wanted to go to graduate school in an engineering field. For this reason I am considering a minor/second major in Maths.

To what extent is my performance in Further Maths reflective of how will I do in a Math major course? Or what Further Maths means to a Maths major course, is just what simple arithmetic means to calculus?