the 'rule' i guess is to have a quarter inch gap at top, but i dont concern over that as long as the shoe fits and is comfortable long term.
saying that if there is no gap that the shoe is too wide, is too general of a statement because it really depends on the make and design of the shoe.
a shoemaker who believes in a quarter inch gap may construct the leathers around my 10D shoe to have a quarter inch gap. if i put on a 10.5 D shoe of his , it may just well close up at the top.
but another shoe maker who does not concern over that, may make my 10 D shoe to have it close all the way at the top, and if i wear his 9.5 D maybe it may be too small and there will be a quarter inch on top.
so you cant say anything about what is correct in general, just case by case according to the shoemaker and how they constructed it.
actual gauge is not the superficial look on top, but the way it fits around the ball,instep, and arch of foot.
you can have a quarter inch spread , but the fit around your feet mayb e too tight.