[lunarhighway]

deflectors are a tricky matter once you get into them. they're easy enough to make, but how large they need to be depends on the particular layout of your car.

from what i've observed on production cars, is that appart from shielding part of the tire they're also used to keep air out of the wheelwell behind the tire. due to steering clearence of the front tire this can be quite a big gap. also suspention links often project from below the lowest part of the stock airdam/bumper/bellypan

what i find very usefull is to look underneath the car at this level and see what sticks out. have a good look at production setups and try to figure out why they look the way they do.
a casual stroll trough a parking lot will be helpfull and unless you get down on all four's to look underneath cars noone will notice... and even than... if you vaguely know people with an interesting car, i'm sure they'll let you have a more upclose look if you express your intrest