Subwoofer Frequency Response question

does the size of the suckout change predictably as Fc changes.

The suckout will be less as you get further from the rear wall (fall off with distance - double the distance = 6 db SPL drop in the strength of the rear wall reflected wave).

Bear in mind that there is more than one suckout - it actually looks like a "comb filter"...each succesive dip as you go up in frequency is lower (this is because less and less energy goes backwards as you go up in frequency - and above 600 Hz most of the energy is going only forward - this is because the wavelengths at higher frequencies are becoming shorter than the speaker baffle)

I see why you like the soffeted approach

It is not me. I simply copy what they do in the highest end custom installations in pro studios. What I have done is far inferior to a proper setup designed by an acoustician, like reknowned Roger D'Arcy but it is still a lot better sounding than nothing at all. I share this info becuase I know from first hand how important it is - I have directly experienced the sonic improvement and it a lot more tangible than your common run-of-the-mill tweaks. (tiny cups, spikes, pads etc.)

In a Mains/Subs set up, you will likely have 2 different Baffle to front wall distances. Do you know how to determine which is relevant (or in what ratio relevance can be determined) to the calculation?

You need software to do this. It is much more complicated. However, the effect with a sub is much less critical because it is rolled off above 40 or 50 Hz anyway and room modes can really dominate at such low frequencies anyway. The best thing to do with a large sub is to get a downfiring one and place it up against a side wall. This means you eliminate most of the quarter wave problems by coupling to both floor and side wall. Since it is to one side then the distance from the sub to the rear wall behind your speaker will vary - say 2.5 meters to one corner but 8 meters to the other corner - this in itself will help reduce the coherence of any cancellation or "suckout" (it is unlike the case of two full range speakers lined up symmetrically to a rear wall). Two subs symmetrically placed close to freestanding speakers should be AVOIDED - this is contrary to the popular audio myth of stereo subs (audio is full of myths). Ideally you can also soffit mount the sub and place the woofer close to the floor but it is simply not as critical as the mains because it is normally not placed symmetrically, room modes dominate anyway, and it only handles the bottom octave (no nasty lower mid bass effects).

Conversely, is the idea behind <1m to keep the suckout in the same region as bass peaks in a typical room? Given the variation in peaks from room to room, this strikes me as quite a risky approach.

I see I missed this. The idea of going close to the back wall (less than 1 M) is to push the "suckouts" up higher above 100 Hz where they become less noticeable. As the wavelengths get shorter then the suckout will be progressively less dramatic...

considering good overall seating location plotting and speaker placement for good coupling with the room for flattest response (use your sound level meter and test discs, at very least, also your ears), you should start by issolating your sub from your listening seat(s). You can easily do this by placing your sub where your ears will be in the seating location, and moving around the room down where you are considering placing the subwoofer. Then, you can play some steady bass music through the sub(set crossover to full open), and listen for a solid sounding bass beat. Then, you will measure to confirm. If you then get a solid bass note from 25hz to whatever frequency your sub will be playing up to (I suggest an 80hz crossover as a foundation), you have found a good potential location for the sub! Then, if your mains are playing solid down to their crossover point (with no sub), now it's just a matter of dealing with blending the volume between the speakers and sub, and tweaking the phase on the sub! You should then have no problem getting a good solid response.
Don't sweat it, you can get if you have patience, and go through ONE VARIABLE at a time.
So again, start with issolating the speakers, then the sub, and then work on the level settings. Good luck