Mate, why you adding and subtracting, when you should be multiplying and dividing?
Basically, it's just this:
Today, with AUDUSD at 1.06785 and NZDUSD at 0.8205, AUDNZD is at 1.06785 / 0.8205 = 1.301463.
Yesterday, with AUDUSD at 1.06785 + 0.0108 = 1.07865 and NZDUSD at 0.8205 + 0.0060 =...