being behind someone gives the width of circa the sagittal abdominal diameter
one will find that the distance between the front of Rayleighs chest to the back of Scopper Gaban's back when standing one infront of the other will be approximately 25 centimeters (9.8 inches). This is for a normal 5'10''ish person.
However for OP characters like Gaban and Rayleigh, with Gaban standing right behind Rayleigh, we need to know their height and adjust for this discrepency. Rayleigh is 6'2'', while Scopper is 6'3" seeing as in this picture below he is a hairs breadth above the hair of Rayleigh.
Therefore taking this all into consideration 9.8x2=19.6
therefore the distance from Rayleighs chest, to Scopper Gaban's back is approximately a bit less than 20 cm.
And with that you know the distance between them, when Rayleigh is standing right infront of Scopper Gaban is 20 cm, so the difference between them in fighting is also 20