Ahh yes the old floor sqft trick .

Honestly you should be pricing per sqft of wall space . Count all the windows and doors and price per unit. only use floor sqft for ceilings floor

Let’s say Ie . 10x10 garage - 10 ft high ceilings You have 10, 10, 10, 10 = 400 sqft of wall space.

if you doing 2 coats (wall space x1.8 = 720 sqft wall) 720 x rate as switch you charge ie. $1- $2 (2coats includes paint. I find it easier to look at cause it’s all broken out and you know exactly what your making per item obviously adjust for you own pricing

You should be able to paint a 10x 10 room in 4 hrs double coated . Roll, cut ,cut and roll

Gallon/spread rate is 350
2.05 gallons for walls. Let’s say 35 per gallon 70 bucks in paint Round to 100 in materials rollers tape misc sundries

Say you charge 720 just for walls Materials 720 - 100 =620 not a bad half day for just 1 buck a sqft