Thank you Webnauts, an interesting thread, with much to be gleaned.

Controlled IBLs, quality versus number etc. is part of what we are charting. Along with other options, both as additions to basic beliefs and in case they are wrong.

A little like the redundant tables, we'll never come up with an exact formula, in a field with so many variables. Even if we did, not sure about the lifetime of any benefits, can't escape a feeling that the rate of change within search engines is picking up.