Calculating a scale ratio based on total sqft
I have a printout of a total office layout and know the real sqft for three of the offices. I need to figure out the sqft of the rest of the offices using a scale. I can measure the office LxW using mm on a ruler and find the total mm area of the office on paper. I need to use this mm to build a scale ratio for the rest of the offices.
My thinking, say I know an office is 93sqft, it's length = 15mm on paper and width = 7mm on paper.
15mmx7mm = 105mm on paper...105sqmm -> 0.00113021sqft
(0.00113021:93sqft) -> this should be the scale ratio?
So another office is 16mm X 14mm = 224sqmm -> 0.00241112sqft
0.00241112 x 82285.59 = 198 sqft, right?
This seems way too complicated. Is there an easier way?