Ian Jackson wrote in
:

at any location it could equally turn out to be almost
anything between 'not a lot', and a lot more than 26dB - mainly
depending on reflections.

Good point. In optics reflections wreak merry hell with polarisation (often
intentionally for good purpose) so I assume that it's the same with radio.
Even so it has to beat putting both in same plane, and I'll use no more boost
than will reduce local RF noise acceptably. Actually I have a bunch of PV
panels and such out there on grounded mounts that any major reflections that
occur won't make it into the flat, it's more likely there would be a few
narrow opportunities for a direct signal to get past rather than many
reflections getting here.