I'm sure Pros have a formula. When I wanted to know the same thing I made cardboard cutouts and hung them on string and then fiddled with placement. Dont forget about the ceiling plate.

Is there an additional light fixture over a dining table?

Hi Nicole,
There is recessed lighting all over the kitchen. The island is custom made...and will serve as our breakfast table. I won't have an additional dining table.
Thanks!

This year oversized light fixtures are a huge trend. However, you need to consider the height of the fixtures compared to that of your ceiling. You don't want anything at eye level blocking you from conversing over you island. Attached is something I drew up to scale. I would use what is below and then look at the height to determine which would work better. Or you can post a link to the light and I can take a look at it for you.

If you only want two, I would try to use the larger ones. Depending on the transparency of the drum/light and the height.

Our guide to sizing light fixtures includes advice about island lighting: http://bit.ly/1cuh1Fj Hope this is helpful!