For myself I just use measuring devises to transfer a known demension to material to work on. I can use anything to do this imperial, metric, a stick, just anything handy. I have built many pieces not taking a measurement at all. Sometimes I just work with what I have. I see a piece of wood and think it would make a nice small table top and go from there. I find pieces that will work for legs and some that will work for aprons and bulld it up by eye. Match the legs up match the aprons up and build it. In most cases unless you are working from drawings or have to have a fixed size your eyes and matching like pieces is just fine.
Jim