I like to use the one-third stop scale. I also often don't bother with the numbers, but just think of intermediate stops as "+" or "-" one-third of a stop from one of the whole-stops (e.g., f/5.6 +1/3).

Question: do you really shoot at f/3.5? If so, then just think of it as f/4 + 1/3-stop (remember, smaller f-numbers = more exposure) Most meters are laid out in one-third stop increments so this should line right up with a mark on the meter somewhere. (Shutter speeds are not so easy though... if you're shooting wide open at f/3.5 then you'll have to pick a shutter speed that can be up to 2/3-stop off. You get to decide whether to over-or underexpose).

@keith: I think you've got your exposure backwards -- f/4 is 1/3 stop underexposure from f/3.5 and f/2.8 is 2/3-stop over (assuming that f/3.5 is the "correct" stop).