Calculate Digit Sum below a Given Value

Calculator for the digit sum until a value is reached or underrun. The iterated digit sum (executed several times in a row) is calculated up to a final value. The maximum final value must be greater than 8, if it is 9, it is the normal iterated checksum.

Example: with a maximum final value of 20 and the sequence of numbers 4547457474567, the digit sum is 15.