How to make a Floor Plan

Well my boss thought me the formula how to convert inch into millimeter to make a floor plan for the office upstairs. It is kind of simple the only thing is getting the correct measurement of the office. I had a hard time doing that because of the Heat, darkness, and not to mention it stinks there. It was full of dust and so well back to the topic. After getting the whole place measurement, I had start from a wall lets say 40mm and then devide it with the real measurement which is maybe 150" so it be like

150"/40 = 3.75

Then after getting that scale measurement, use that "3.75" to devide with the real measurement of the office and you get the size in millimeter. Well got to measure the whole place in inc and it be much more easy then to draw the floor plan. :P