When I used to set DCs I'd use a scale of 1-10 x 3. Easy is therefore a 9. I wouldn't call for a 3 or 6 DC unless there's a reason that charavter might fail, and 27 and 30 i would only use for something legendary.
Which mostly leaves 9, 12, 15, 18, 21 and 24 as my main DCs. Gives me a bit more...