The whole deal about solar flux and the A and K indices has always been confusing to me. To be quite honest, I am not really interested in the details - I just want to know when the bands will be open. After looking at various articles it seemed to me the whole propagation predition game is like the stock market - in the short term it might be way off the mark, but in the long terms it does pretty good.
I finally stumbled onto an article that was published in QST in November of 1991 called: "Propagation Broadcasts and Forecasts Demystified" by Rus Healy, N2JL. This article is available from the ARRL website - members only, section. Towards the end of the article there was short summary that told me all I wanted to know:
On 20 through 10 we need high solar flux and low A and K indices. It seems that the solar flux is the important one - i.e. even if A and K is low, but solar flux is low too, you are not going to get anywhere.
On 160 through 40 we need low A and K values and the solar flux does not seem to matter much.
Looking at the columns "10.7" and "Ap" on the NOAA page explains why 20 is very unpredictable at the moment and why 40 is good.
I have been noticing that 20 stays open much later into the evenings and I regularly use 17 in the early evenings - so it is obvious that things are picking up.