The DC to change a target's attitude from Hostile to Indifferent is 25. That's 6 or higher on d20 for our +19 Bard friend, or automatically if she's taking 10.Even if it can pump up to +18 its not an accurate statement. Hostile isnt restricted to a specific range. Furthermore, some npcs can have personality differences that increase or decrease benchmarks fir such things. ALSO hostile verses non hostile is not the only difference between success and failure.
Sure, you can add lots of extenuating factors, but that sort of renders the Diplomacy DC table rather pointless, doesn't it?
As a baseline, it is not desirable that a 2nd-level character can automatically make DC25 skill checks, or a 3rd level character automatically make DC30 checks.