The rated capacity of RDs is usually found on the maker's website. However, the rating isn't relevant to the conversion, which you're doing by direct observation. The only use for the rating is to predict the outermost rear sprocket you'll probably be able to use with the inner.

BTW- you earlier asked about the odds of needing a longer (new) chain. Going up 2 teeth or so, you can usually get by, but don't bet on it. More than two teeth odds are you'll need the new chain.

You can predict this by shifting to the big/big combination, then pulling up the lower loop, each 1/2" you can pull forward is worth 2 teeth on the largest sprocket.