For a long while, I know that professional framing people always get framing ready laying on the flat ground surface first before erect it into position. Since I am not sure about my measurement, I always build things in 3 dimensional space using screws. This way, I can make sure everything falls into their proper places. If not, I can un-screw it and do it again.
But, everything has its start. Therefore, while making the shelf for four plastic storage containers, I decided to follow what professional always do. Get it ready on the flat ground surface first – I made two identical longitudinal framing with five (5) 2×4 first. Then erected it. The result is pretty good I think.