I have the vaguest concept of the meaning of these indices. I read
somewhere that
a general rule is if the A index is 15 or less and the K index is 5 or
less listening will
be ok. But I've never seen a guide about Solar Flux numbers. What's a
"good" number
for SF?

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A lay persons guide to Solar Flux Index

SFI = 60 Lousy - The Pitts
SFI = 80 Grim - Go to the lower bands
SFI = 100 Ah lookin better but don't write home bout it
SFI = 140 Now its cookin
SFI =180 WOW -- hello world
SFI = 280 Gandbusters a wet noodle antenna will do ya
SFI =300 My prayers have been answered

But depends on the frequency and the A and k indices as well.

See URL's
http://www.wm7d.net/hamradio/solar/summary.shtml

AND

http://members.cts.com/fort/w/wn6k/sf2.htm

--
Lamont Cranston

The Shadow Knows