Just upgraded to General a few weeks ago and starting to explore HF. I keep seeing people mention solar flux index and K-index numbers but I'm getting confused by all the different values. Current SFI is around 135-143, which seems decent, but how do I know if that's actually good for working DX on 20m?

Also seeing K-index at 1 which people say is "very quiet" - does that mean good or bad for propagation? Any simple rules of thumb for a new HF operator?

Welcome to HF! Generally propagation conditions are OK when the A index is 10 or lower, and the K index is 3 or lower and the SFI above 90. Your current numbers sound great actually - K index of 0 or 1 is best, 2 is OK, 3 or more not so good. SFI in the 130s will give you solid 20m conditions during the day.

I was in the same boat last year! One thing that helped me was bookmarking a propagation site and checking it before getting on the air. Focus on 20m, 17m, and 15m during the day with those numbers. Don't get too hung up on perfect conditions - I've worked great DX when the numbers weren't ideal.

Digital modes (FT8, FT4) will help you snag weak-signal DX, especially on 10m and 12m when conditions aren't perfect. Even with moderate SFI, FT8 can pull signals out of the noise that you'd never hear on SSB. Worth trying if you haven't yet!