The Ideal Height
The ideal bar stool should be approximately 28 to 30 inches high to match the 42 to 48 inches standard bar height.
There is Room for Variation
Because there is indeed some variation in bar height, there will also be variation in stool height. Therefore, depending on the exact height of the bar, the stool height should could be a little shorter or a little taller than 28 to 30 inches.
Although you can expect your bar stool to be approximately 28 to 30 inches, you should realize that ultimately it is determining the stool's best height in relation to the bar itself that matters.