Swatch! cables change the gauge.. (plain stocking knit gauge of 4 stitches to the inch/16 to 4 inches (worsted weight) becomes
2.5 stitches to the inch/10 stitches to 4 inches with a simple 'cable' (4RC) complex cables can change the stitch gauge more!
so its not easy answer..
you can think about adding a pocket use your swatch as a pocket--but the only way to know how many to cast on, and how to space the cable is to swatch.
for an interesting (ie several styles of cables) pattern, it's not uncommon to start with a 100 stitch swatch, (and it's not uncommon to make 2 or more swatches.)
cables are pretty easy to knit (and to chart) but to design a well fitting cabled sweater swatching is MUST!