There is no magic formula/answer.
But, since most gauge swatches are based on the number of sts to get 4", you should probably CO more sts than that.
I usually CO 1.5-2 times what is reported as being needed to get 4" and knit away.
Too few stitches and you won't be able to measure 4"
Too mant sts and your gauge swatch might as well be your project.