i think a part of the reason this is confusing - and the survey states as much up front - is that the language they're using is often inexact, to say the least. the ideas are not being presented clearly. so, let me work through this and try to get the ideas clarified, first.

their argument is that cysteine can "substitute" for methionine if it's present in sufficient quantities that you can prevent the conversion of methionine to cysteine. but, this isn't actually a substitution process at all. it's more like a blocking process. they should by talking about transsulfuration-blocking, not cysteine-sparing.

they then erect this idea of "total sulfur requirement", which is the amount of methionine you need without any cysteine, and subtract out the minimum obligatory amount of methionine, which they decide is the amount of methionine you need to do methionine things. the argument is that what's left should be the amount converted to cysteine, but, as i've said before, that doesn't actually make any sense, and i'd advise against citing that deduction. i guess you could use that number as a crude upper bound, but you don't actually know how much non-essential methionine gets converted, so you can't actually say anything besides that. the amount of cysteine you need to block conversion could very well be half of that. worse, if you go back to the first study, it mentions that they don't know that the amount of methionine cited is truly sufficient for what i'm measuring - they explicitly poi\int out that that number may be insufficient to produce enough taurine and glutathione. so, you can only deduce that you need some amount that is less than 10.9 mg/kg of cysteine to block transsulfuration from occurring at levels that may or may not be sufficient to meet cysteine needs.

so, what's the right experiment, then, even if i can't find it?

what you should do if you want the answer i'm looking for is measure the maximum amount of methionine that gets converted to cysteine, and base your cysteine requirements on how much you observe your body transsulfurate. so, what you want to do is give the subject massive levels of methionine & serine with zero cysteine and zero cysteine derivatives and see where the breakpoint occurs. that will determine total dietary cysteine requirements, independent of methionine. i'd want to take that number and build an rdi for cysteine on it. then, i could subtract that out from the total sulfur requirements to get a methionine rdi.

the confusion is likely stemming from methionine being seen as essential and cysteine being seen as inessential. that may be technically true, in terms of the chemistry, but cysteine seems to be the more valuable chemical, so it should really be what the requirements are built around. methionine may be indispensable, but only at much lower levels, and only as an after thought, in the presence of sufficient cysteine.

so, i'm taking a giant step back and asking a different question - has anybody tried to measure how much total cysteine your body requires, independent of methionine? let me figure that out first...