Don't think of this job as per-foot. This is a repaint with extensive prep so you will need to look at all parts and price accordingly.

Pricing per sq/ft only works for new construction where you will be bidding off prints and know what to expect. For this job you have a lot more to think about and take into consideration;

1- Possible lead paint dust. Some containment to control the dust from the prep will be needed regardless but with EPA lead rules much more is needed. Are you EPA RRP certified?

2- Amount of furniture you need to move and cover. Never underestimate the time it will take to move and cover everything just to get a room ready to work in.

3- You will be priming all surfaces. Make sure to add enough for the materials and labor needed to prime everything.

4- What paint will you be using? Cost are all over the place for paint but you are in the right place to give options. Not just low or no VOC, how about non-toxic? Mythic non-toxic paint, Sherwin Williams Harmony, Benjamin Moore Natura or Aura are all fine options for your customer to consider. They are more expensive but offer a better longer lasting look without any fumes.

All of these things and more come into play when trying to figure out a price. This is why I don't believe in per foot pricing for repaints.

Hope this helps.