==" just put it short,
for a R6 to get to R10 , 4 rank jump is needed = +4 .

If u compound 2 different rank like R3 and R6, the lower rank will be the one considered how many rank to jump to, Example R3 + R6 = R10 , +7 jump is needed.
If it drops in rank, it will be consider as negative = -1 or some negative numbers