I can't really tell you exact volume settings, but I can give you some tips. Regardless of miked or unmiked, I recommend using an amp stand. I particularly like the one from Gruven (www.gruven-stands.com) This raises the amp off the stage, angles it towards you, but also helps it project out into the room. I find that it kinda bounces it off the ceiling in some clubs, instead of shooting it at knee level, blasting those in front, and muddying the sound for those in back.
I usually recommend mic'ing regardless of the size of venue. It just gives you an extra measure of control.
Hope this helps.
RB