Indeed, the formula is correct. Wikipedia maintains a list of historic Forever pricing: https://en.wikipedia.org/wiki/History_of_United_States_posta...

From the blog, the letter from California was dated April 2022, at which point the rates were domestic = $0.58 and global = $1.30. So the California sender correctly attached two domestics valued at $1.16 total plus an additional $0.14 to make $1.30.