If you use less stock per roll than the amount recommended by Kodak you aren't risking problems with overall contrast - you can correct for most of that by extending development.
Your problems are most likely to be related to localized developer exhaustion. If you have a bunch of high key images, or others that would lead to dense negatives, there may not be enough chemical activity available in your developer to fully develop all parts of the negative.
Other developers have more chemical activity available in each millilitre of stock, so you are less likely to run into the problem.
The volumes printed on the bottom of the Paterson tanks only indicate how much liquid will be necessary to cover the film. They say nothing about how much developer is necessary to fully develop the film.
If you use less stock for each roll, it will work inconsistently - full development for some rolls, slightly incomplete or uneven develop for others.
Kodak builds extra safety margin into their recommendations - but I am inherently conservative in such matters, so I follow them.
Considerations like this are why I recommend a replenishment regime.
Kodaks D76 data sheet text which I have a PDF of...
'
Dont reuse or replenish the diluted
solution. You can develop one 135-3 roll (80 square inches)
in 473 mL (16 ounces) or two rolls together in 946 mL
(one quart) of diluted developer. If you process one
135-36 roll in a 237 mL (8-ounce) tank or two 135-36 rolls
in a 473 mL (16-ounce) tank, increase the development time
by 10 percent (see the following tables).
'
It does not say 473mls is a minimum for complete development, badly written (I'll give you) eg first 135-3 should read 135-36.
Ilfords data sheet for ID-11 is better written and additionally provides times for 1+3.
The other clue is using a litre of stock you can process 10 films (135-36) so there is enough capability in 100 milli litres of stock for one film.
My Agfa daylight loader only needs 200 mls {or less) for 35mm (135-36), Ive done 1+1 ID-11 in it with no detectable contrast difference, but it does need continuous rotation.
The only problem with using 1.0l stock for ten films (Ive found) is needing to filter 'debris' from earlier films, which is just as likely with replenishment... I use a coffee paper filter.
Note most of my film is Kodak cine negative, and when I use D76 it is a scratch mix D76d, pH buffered for stability/longevity, Kodaks commerical packing may be very similar.
If you are not getting contrast for grade 2 - more time is needed.