It has to do with the fact the strips are cut in half after each row (or rows) are sewn together. So, the way I do my calculations is this way (I'm math challenged, so there might be a better way). In this example, I want the quilt without borders to be 60" wide:
60 -- 2" long (finished) -- single strip before doubling and sewing)
120 -- 4" -- 2 strips sewn together
240 -- 8" -- 4 strips
880 -- 16" -- 8 strips
1760 -- 32" -- 16 strips
3520 -- 64" long -- 32 strips
So, you start with strips sewn together equalling 3520" -- about 83 strips, and you end up with a quilt 64" long. Probably not long enough but unless you want it to be 60 x 108, you can't just double the length of the beginning strips in the last line. So what I do is decide how much longer I want it and make the strips for that length. So, I've decided I want the size to be 60 x 72", I need a piece 8" long. So I start over with another 6 strips (240") and then sew the 8" piece I end up with to the main quilt. It won't be a perfect match -- I usually have to trim an inch or so off the width of the added piece, but it's close enough for me.
This is probably as clear as mud, but it works for me. The trick is to decide on the width you want, calculate the number of strips to gain that width (I divide by 42" since fabric is 42-44 after selvedges are removed), then do another piece to sew to the main one to get your length.