You can do it any way you want. I row is 2 inches finished. How many rows do you need to get the length you want? Just plug the numbers you want into her formula. She is leaving room to add borders so keep that in mind if you want borders. Here is an example.
80 x 90
80 inches x 45 rows / 41 inches per strip
80 x 45 = 3600 inches / 41 per strip = 87 strips needed