I read some Kodak data sheet once that said 6 mL is the recommended minimum, but only 4 mL of syrup are actually used up by a properly exposed average roll developed normally.
I have used three mL per roll and had no problems achieving normal contrast, though.
One of my thoughts on the issue is that manufacturers' stated minimums can be halved without any consequences; you can achieve normal contrast, and in all other ways get a negative that is just as properly developed as you would get from meeting the stated minimums. This is just based on my own experience, and may not be true with some oddball developers, though.
The other of my thoughts on the issues are that one who is using highly dilute developers is perhaps (if not likely) doing so in order to purposefully "suffer" some of these consequences anyhow; the stated minimums are for those seeking standard negatives developed to a similar contrast as that achieved by Kodak in its testing, but for those looking for something else, breaking the "rule" here is a special-purpose technique.
240 mL/64 = 3.75 mL, so using dilution H with 240 mL per roll is safe without a question in my book.
FWIW the guy who wrote The Darkroom Cookbook is fairly adamant that one should use 500 mL/16 U.S. oz. and no less to develop a single roll. For PMK pyro, the Formulary data sheet states the same thing. This does not improve my negatives in any way, shape, or form that I can see, and I personally find it to be quite wasteful – mainly of time, but also of chemistry. I don't do enough processing as it is; having to do twice as much would be "disastrous," in terms of my stockpiling of exposed film to be processed.