Bias cushion cover

I’d like to knit a stockinette cushion cover on the bias. But, I am having trouble figuring out how many to cast on, and how often to add stitches, and how many to add each time. It’ll be the same situation at the halfway point when I will need to decrease. The cushion cover measures 15" x 15". Thoughts? Thanks!

It seems to me that you could use the patterns from these baby blankets as a guide.

This one starts out with one stitch and increase either at the beginning of every row or at the beginning and end of every other row.