It is the ratio that is important here, you need to create the right strength of solution, it is not not 250ml specifically. I actually did the math originally to get the numbers for a full liter but in my Nikor tanks, 250ml covers one roll, 500-2, 750-3, so I just use/kept those numbers.
The place where 250ml is important is that it is the smallest amount of the normal working solution needed to develop a 36 exposure roll of 135 or 1 roll of 120.
Much depends on your tank but your number sounds too high. The tank does not need to be full. In fact I purposefully leave at least one reel out and only fill the tank enough to cover the reels. This leaves a significant air space at the top which I think helps the replenished bleach work better, it needs aeration. Given that the developer is typically mixed moments before use and then immediately discarded, maybe 15-minutes total exposure, the air doesn't have time to affect it.
To find your real numbers throw a reel in your tank and then fill until the reel is just covered/submerged, dump this into a graduate, remember that number; that is the minimum amount of chemicals I would use.