Most exterior paints will cover about 400 square feet per gallon for one coat. But adjust for waste factors like the type of surface and equipment used. If the surface is rough or porous, add 20% to the total gallons figured. If siding is narrow (4 to 5 in.), add 10%. If on a corrugated surface, add 33%. Also add on a 10% waste factor if using brushes or rollers, add 20% for airless paint sprayers, and add about 40% for air paint sprayers.
The problem: you don’t have a mortar box or wheel-barrow to mix up those small batches of concrete. Instead, use the ring method on a slab or sheet of plywood. Put down coarse aggregate on the bottom, followed by layers of sand and cement. Reserve the center for a mixing well. Then add water to the center and pull the ingredients into it with a garden hoe.