I'm a little confused, which isn't all that uncommon.

I just installed a Cateye wireless computer on one of our bikes, and couldn't find any mention in the documentation about placement of the magnet and sensor. Seems to me they should both be as close to the rim as possible, but this model has a sensor mount that puts it about 3 inches below the rim. Seems to me the closer both are to the hub, the more it's going to distort your speed, mileage, etc.

Is there a rule of thumb on the best place to mount these parts, like xx inches from the rim? And if it's in the wrong spot, will your numbers be appreciably affected?