The standard answer is 1/2″.
If your shirt doesn’t set up properly under a suit, the only solution is a custom shirt. When you get measured for a custom shirt, your tailor will assess the length of your neck, and design your shirt accordingly, adding or subtracting height from the collar’s band to create the perfect collar for you.
Unlike other aspects of your clothing where ready-wear clothing can be modified at a reasonable cost to mimic custom, altering the height of one’s collar is not an option.
Most guys show 3/4″ of shirt at the very back of the collar of the suit jacket. I’m not sure why this is, but suspect that many shirts are made for a more casual look…therefore somewhat higher than a dress shirt needs to be, and when worn with a tie, result in slightly more reveal at the collar than necessary. Anything more than 3/4″ is unacceptable.
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