The answer is squarely in the middle of "it's subjective" and "it depends".
General rule of thumb... if it's a larger mass, a long smoke, and/or a product where most tend to want a heavier smoke flavor (brisket, butt, turkey) then you use 4-8 oz of wood. If it's something delicate (fish), a small mass (a chicken), or a pretty short smoke time then you might only use 1-3 oz of wood. For variables in-between you do the 3-4 oz. of wood. Of course, you increase or decrease to your own personal preferences.
Perhaps the rule for your other thumb is, if in doubt, use less wood... it's rare to make something inedible because you used too little wood. Sure, perhaps it could have been better, but it'll be edible. The forums are littered with posts where too much wood was used and the flavor was too strong and/or bitter and the food ended up getting chucked.
You mentioned brisket... If I do a 12+ lb. full packer then I use 8 oz of wood... typically hickory, which is considered a stronger flavor wood. I've never done a smaller brisket, but if I were going to then I'd probably use 4-6 oz, instead.
