This is an interesting question, in that, at least in my opinion, and if I understand it correctly, indicates some rather strange but certainly understandable thinking.
The tool of controlling heat inputs is not so much base metal related as it is resulting micro/macrostructure, and therfore corrosion or mechanical property, related. And while the type of base metal under consideration is involved the heat input is tied more closely into these resulting properties of the base metal.
For example, you would not necessarily ignore the fact that you are welding either 316 SS or SA-333 Grade 6 CS, but a 50 k/j max for SAW could be the optimum (or acceptable) for both applications.
So you 'sorta' look past the base metal and ask yourself what it is you expect of the base metal.
And it is not always the highest joules you want to consider. This I believe is code think, driven by code requirements. What is most important is an evaluation of the material you are working with and what you want it to do. Sometimes the heat input needs to be a min(I would be more concerned about reducing grain size in high temp creep applications where large grains are an advantage than I would be for reduced grains for toughness), sometimes a max(as in impact toughness regimes), sometimes a window (as in Duplex SS's). But codes normally think in terms of max's because the heat input requirement is, to my knowledge exclusively related to impact test regimes (though AWS D1.1 certainly runs right up against a concern for non impact regime heat input control without being explicit, by its % controls over amps, volts, and travel speed) wherein for most alloys toughness is improved by keeping grain size down.

This is a question that has me baffled also. Fortunately, this data is well documented for stainlesses, and some info is out there for T1, P91 and related CrMo's but is hard to find.   I have searched the web back and forth and called many steel mills for this same info for many alloys I commonly work with (41xx & 43xx) and to no avail.  You may want to search out this Forum for posts by "Stephan", as this is his favorite subject and he makes mention of the "t8/5" parameter which may get you to the same place for calculating max heat input. I have found that many European mills have this type of data on their alloys; unfortunately European alloys are not very similar to what i have occasion to work with,

What specific alloy are you requiring info on?

I can tell you this, my search for the same info on 4130 was totally frustrating, so I used a number of 55,000 J/inch for PQR's I am doing right now. That is a number I saw referenced in an article on P91 in "The Fabricator".  This seems to be have been a very good target for me although I have not gotten test results back yet but I can tell it is just right (good flow, no distortion, mag perfectly).  I hopes this helps as a benchmark for what you and others may need to do. PLEASE, if you or anyone knows more about this subject and where to get good numbers, please post.