Stretched swingarm/ Chain length

Okay, so I've got a 2007 sportster, converted to chain. With a 23/60 sprocket combo I'm using like 118 links of 530 chain on a factory swingarm.

I dropped a swingarm off today for a 7" stretch job. Is there a mathematical formula for figuring out how many links I need for a new chain, or is it estimation guesswork?

Thanks.