I have my original service sheet for the RD-6400.
It says:
-front difference, 13 teeth or less
-total capacity (front difference plus rear difference) 28 teeth or less
-rear smallest, 12 teeth/rear largest, 28 teeth
with B-tension adjustment completely tightened.
It would appear the OP's range is OK with a 12-28 (16 difference) and 42-53 (11 difference) = 27 teeth.
Here is the sheet showing the B-tension adjustment and correct chain length.
RD6400