You could make the stocking as is, but it will come out larger than the original. That would be the simplest, and you could follow the pattern exactly.
Because of the shaping involved, it would be difficult to give an exact math formula.
If you want the stocking closer to the size of the original, you could subtract say, 10 stitches from the center and keep the shaping the same.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry