My car has tires 215/45R18. I want to get the 225 width size. Using the calculation if I put 225/45 R18 the tire diameter increases by 1% and with 225/40 R18 it decreases by 2%. What effect would the different sizes have on acceleration / handling?
From your numbers, it would be negligible.
Thing is, though, you're measuring it the wrong way. Because every manufacturer and even the same manufacturers don't measure there tires the same way by width and aspect ratio. The numbers can be off by (at a guess) +/- 5-8%. The better way to get a more accurate tire measurement is to look at the rotations per mile. Most good tire sites will give you this number for the exact tire you're looking at. If you compare these two numbers you can see what you're coming from and what you're going to and come up with a percentage difference. Then you'll have to take this number and figure out if its something you can live with or not.
As far as acceleration, a smaller tire will give you faster acceleration, but cut down on the top speed. Opposite would be true of a taller tire. As far as cornering, the wider tire might give you a little extra gription, but really, it wouldn't be anything you could detect by the seat of your pants. Vice-versa is true as well.