There must be a formula!

Hello and Happy New Year to all!

I am writing to ask about sweaters. What is the formula used to turn a pattern for a pull over sweater into a cardigan sweater? I know it must be fairly simple to open the pull over and make it a cardigan. Any ideas about how to calculate, or what the process is?

Thank you,

Are you planning on making it one piece without seams or knit in pieces and seamed together.