There is no "standard" height for wainscotting but somewhere within a range of 28-36" is common. You might want to look at window heights as you make your determination as it can be tough to run the wainscotting at the same height as the window sills. Same issue with countertops. Find a height where you will not have problems joining moldings. With your high ceilings, you may want to go toward the higher side of the common range for aesthetics.