How do I determine the ideal height stool for a given height surface (table, counter, bar, etc.)?
There are common established heights for table/counter surface and seating combinations. They are:


Table height: 30in (750mm), with a chair at 18in (450mm).
Counter height: 36in (900mm), with stools at 24 in (600mm).
Standard bar height: 42in (1050mm), stools at 30in (750mm).
Extra tall bar height: 48in (1200mm), stools at 36in (900mm).


(Source: runmyhouse.com)

In all these combinations, the difference in height between seat and table/counter surfaces is 12in (300mm). Whether or not this is ergonomically ideal, most people will be used to these sorts of dimensions, so sticking with them is a safe bet.