This app was mentioned in 3 comments, with an average of 3.00 upvotes
Correct, whatever the focal distance is, not the 35mm equivalent. The app will do the rest for you. I use Lenslab if you want to compare your results to another app's results.
Lenslab because it was made by a redditor and it's easy to use.
I use Lenslab on my phone and love it.