If I was to go from a 40 tooth largest cog to a 46 tooth, on a 1X setup, how much longer does the chain need to be?

I知 thinking 3 links. Am I correct?

There are other variables that come into play. Single speeds for example are different than derailleur bikes. Try 3 and make it longer or shorter an necessary. I usually just start with a new chain and shorten it as is necessary. Roger

Yep. A half inch per tooth.

Try the chain you're using with it now and see if the length works. It may not, but the exercise will help you visualize. If, on your large rear cog, the DR cage is stretched to the max, then add a link or two. Other wise leave it alone. On a 1x, a shorter chain will assure better return-side tension that will reduce the chance of a chain drop. When I went from a 39 to a 42, I found that I needed no additional links.