Yep. Used this calculator last week for my first AG and it worked like a charm. For your 12lbs of grain, qts/lb of 1.25(average), TOTAL space used is 4.74gal making a 20qt(5gal) mash kettle/vessel ideal(minimal headspace equals better temp retention during mash)
Oh, seek out Deathbrewer's stickies for BIAB/AG BIAB in the Beginner section.