The calculator applies the same Pythagorean letter-to-number chart used for other name-based numbers, but restricts itself to consonants: every letter that isn't A, E, I, O, or U (with Y treated as a consonant here, since it's reserved as a vowel for the soul urge calculation instead).
Because the process is entirely rule-based, there's no interpretation happening at this stage — just the same arithmetic a person could do with a pen and a printed letter chart, done faster and with less risk of a dropped letter.