The maximum number of Volume that count PER leg for the next rank is 40% at anytime
You can take the total Volume to reach a rank and divide it by 40%, that will give you the total Volume that count towards that leg.
example: if you are a Platinum 600 running towards Platinum 1000: Platinum 1000 requirement is 4,350 in volume.
40% of 4,350 = 1,740
Therefore the max amount of Volume on one leg that can count is 1,740 in volume.