How to calculate largest knittable square from cake?

I have asingle cake that I am wanting to turn into a shawl, and for that I want to knit a giant square, while i have the swatch size (22st, 30 r for a 10x10 cm square) and the length (940m) I am wondering how I can calculate the largest possible square I can knit with this as I only have the single, I dont want to waste it as its an alpaca mix and quite expensive.