Brian,
Sq. Ft. bids vary widely and are subject to many different factors ( including experience ). I suggest that you start out by charging an hourly labor rate plus materials (or let the customer provide the materials.) Do not sell yourself short, charge a rate that you can live with.
Offer a general idea of how long you think the job will take, this will give your customer a ballpark figure of the job's cost.
Establish a reputation for quality workmanship while you gain "on site" bidding experience.
Admin propainter