Ok it's clear that is the mix of all the boundary that determine the center frequency of the cancellation because in a small room all boudaries are close enough to always play a role in the equation.
Well the size of that 'role' seems to vary a lot and not intuitively. IMO certainly enough to warrant separate identities to the boundaries to prioritise treatment.
A Real test in a Real Room shows the Back Wall contributing a lot more BIR than it's distance would suggest. SBIR
On the other hand FWBIR can be pretty much eliminated by moving the speaker very close to wall with an absorbent panel between. SBIR
I have another test with the speaker in a tricorner, equidistant from all walls.
When moved towards one wall, keeping the others the same, the combined null does appear to shift frequency. So I guess the influences kind of blend together in that close instance.