I don't quote prices over the phone or internet, if I get asked for a quote, I say I need to see the carpet in person before I can give a price.
I charge a minimum amount for up to 20m2, then £x per m2 on top of that, a second room I have a lesser price for, up to 20m2, then the excess if it is greater than 20. A third room onwards just charge a flat rate per room, so the first two rooms are charged by area,I always charge by area for the biggest 2 rooms, then after that its per room, with a 3rd or 4 th room being discounted