This depends on whether you're building furniture for a general population or a specific individual. An individual can tune their furniture to their body. If this is the case and the furniture is for a specific person, you can follow advice from http://woodworking.stackexchange.com/questions/1667/is-there-a-procedure-or-equation-to-calculate-optimal-height-of-a-seat-based-on as others have suggested, or you can simply work with the person to find where they're comfortable, by visiting a furniture store or having the person sit in an adjustable chair to find the height they're comfortable at.

If you don't know who will be using your furniture, you need to go for the average person to make your table fit the largest portion of the population possible. To that end, there are "approximate" standards, which are suggestions and an inch or two in either direction won't have a significant impact.

A typical dining room has a table at 30 inches and a floor to seat height of 18 inches for the chairs. A typical desk height is 29 inches, and a typical office chair is adjustable, but extrapolating, an average desk chair height would be 17 inches.

More generally (from [How to Choose the Right Size Dining Chairs][1])

**Regarding table and chair height:**
> Allow 12 inches between the seat of your chair and your tabletop.
> 

This in particular is the answer to your question. Given a table of x height, the chair seat should be approximately x-12 inches. I would not go with a smaller distance, but one or maybe two inches lower would be OK.

>Leave 7 inches between the chair arm and the bottom or apron of the
> table.

**Regarding chair width:**

> Provide each guest 24-30 inches at the dining table. 

> For rectangular tables, add an extra 12 inches at each end for those seated  at the head and foot of the table. 

  [1]: http://www.wayfair.com/Buying-Guides/How-to-Choose-the-Right-Size-Dining-Chairs-E568