Even I, mired in the throes of dyscalculia, can look at those numbers and know I need more material if I'm going to fully cover the windows. However, after putting up two test panels on the flanking windows, I realized that for the center window I am not going to want full coverage, or (among other things) the hummingbirds at the feeder will ALWAYS look like those grainy blurs I tweeted earlier. So my plan became to fill in the lower portion of the two side windows (a 19"x 21" space on each), and if the numbers work out, order another roll to cover the center panel leaving a viewing gap at the top.
I am terrible at arithmetic. I used to think I was just bad at math, except that didn't seem entirely right because I got mostly A's in math classes until I hit a wall in trigonometry/pre-calculus. Knowing what I know now, I'm pretty sure it was the sine/cosine/etc. tables that did me in there, since this was just prior to computers taking over the world, and certain sadists in the field of childhood education felt that the use of calculators was tantamount to cheating. Ahem. In any case, my problem isn't with theory - my problem is that in my mind, numerals and numbers both behave like they are made of mercury, and the moment I turn my back on them they dissolve into a giant puddle of "set it all equal to 1". Plus the classic mistakes of 2 for 5, 6 for 9, 3 for 8 and the reverse.
Practical projects go something like this: measure window, write down measurements, measure window again, write down measurements, add 36"+ 19" and get 45", do some area calculations, realize something is off, check my math with the calculator, discover that 36+19 is 55". Wonder how I both carried the 1 and dropped it at the same time. Swear off doing math in my head AGAIN, except this was a sum I'd worked on paper. Twice. Remind myself this is why I check my arithmetic with the calculator, especially before I start the part with the slice-y things. Go online, order another roll of film; cut the remaining film I have to size and successfully apply it.
I like the feeling of accomplishment I get from rectifying household issues like this one, but sometimes the figuring takes three times as long as the actual work. Good thing I can judge success by what works.