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.
When cutting up small pieces for a project, an easier way to doublecheck for accuracy is to measure a number of the pieces at once. If you have four pieces which should be exactly 8 in. long, lay them end to end and measure. This multiplies small errors hard to detect on only one piece. For example, if the four pieces measure 32-1/8 in. instead of 32, then you know that each of the pieces is 1/32 in. too long.