It makes no difference where the fuse is to start, as long as it is held firmly in place so that you may move the incidence meter from one surface to another.

Take a reading on the top wing. Let's say it shows 7 degrees positive. If the bottom wing also shows 7 degrees positive, the wings are parallell and at 0, 0 incidence to each other

If, on the other hand there is a difference in the numbers, then there is a difference in the incidence and you will need to adjust one wing or the other.

The same is true for the engine and stab. If, in this example, if all are at 7 degrees, then the incidence is 0,0,0,0

JR