A good measure to go by is SL /10. For instance, at SL 20, a +2 weapon, or SL60 a +6 weapon, up to SL ~90 which you can have a +10 weapon. Estus is not kept track of for summons, so you can have 15 estus +10 and invade someone at SL 1 at the The High Wall. Another good measure to go by is checking your own weapons and SL at each area in the game to see where most people would be. You can invade and be summoned by around +/- 10 levels, and +/- 2 weapon levels.