Yust screw it as close as possible,that it rotates comfortable when turned by hand and at the same time dont spill the developer if you lean the unit farward while rotating the rollers.
Yust use your feeling.
Put it in,make some copies and inspect if everything is OK.
Adyust if necesary
Go with the 1/2 mm setup. Ricoh has a Paper Dust Cleaner (A2949560) that fits between the Mag Roller and Doctor Blade for cleaning purposes, so it has to fit along the edge of the Doctor Blade. I think Richo-ono meant .5mm; I know when I am clening along the Doctor Blade I fold a sheet of paper in three and it fits in along the opening.