Over several years of experience with different players, different in body height and different in playing style, I defined a standard string spacing for all my instruments. This spacing is progressive, increasing from the high to the low strings. The spacing is not traditionally measured from mid of each string to mid of each string.
On my bass guitars the spacing is measured from outer edge to outer edge of each string. This is mathematically not correct, but you have a real better feeling and control while moving with the right hand from string to string.