Math, not being one of my strong points, I usually avoid all things with numbers and formulas. Unfortunately, sometimes one just has to bite the bullet and do some simple sums and measuring. There are several ways to determine the correct size for a dining chandelier.
How to hang? First, use you instincts. Your guests must be able to see each other across the table. This means that your fixture should hang no lower than 30 inches from the top of the table. The usual range is 30-32", escalating to36 inches. It also needs to be dangling at least 2 feet from the ceiling, assuming 8 foot ceilings and adding 3 inches for each additional foot of ceiling height. Sometimes narrow rooms are a problem. Be sure to keep you chandelier a minimum of 4 feet away from any walls.
How wide? You have two options here. You can use the 12 inches less than the width of the table or 3/4 width of the table, but don't worry about an inch or two excess. You do not want a skimpy chandelier. I like the formula that adds the length and width of one set of walls (ex. 16+18) and the total yields the correct diameter for your lighting (34) ...just like magic.