Here's a link to the manual for a 3 Pro. It has lots of frequency info in it. http://www.ampeg.com/pdf/svt-3pro.pdf
Here's what I like to do (after years of learning the hard way). Start off in every room kind of flat (and in your case without the graphic EQ engaged). Now, every amp will have a little bit of "color" to it when set to flat, but it's a good start. Dial in "your tone" for that room. Different stages and different rooms will sound different. The only caveat to that is if you are using PA support, be sure to check with the sound guy as well. He may need you to stray from "your tone" just a little if it's too muddy or too bright for the FOH mix.
And in your case, after you have dialed in WITHOUT the graphic, then punch the graphic EQ button and set yourself a 2nd tone. That way you can use one tone for fingers and one tone for pick. Or you could use one tone for finger funk and another for slap. You get the idea.
Short version. Don't get too caught up in the numbers. Use your ears. Boosting 300Hz means nothing if it sounds like crap.