Sounds like your boil of rate is pretty high. Take two gallons of water and boil it and after an hour measure what's left...what boiled away will be your boil of rate per hour...adjust your starting volume from there. And take grain absorbsion into consideration as well.
EDIT: Simple BIAB Calculator.
Bottled: Mexican Cerveza, Irish Stout, some Blue Moon clone, Pepper Ale, Full Hard Lemonade, Russian Imperial Stout, My first attempt at Mead (JAOM), Black Pearl Porter
Kegged: Blue Moon clone, Irish Red Ale
On Deck: Citra Hop Bomb (small batch) and Cream of Three Crops
Never underestimate the power of stupid people in large groups.