you cannot do that with sprintf at all unless you are certain that you are using aa fixed width (monospaced) font.
If you are using aa fixed width font then code the other text excluding the signature as cell array of character vectors or as aa string array. find the display length of the longest line (tricky if you have combining accents or zero width characters.) Now display the text such as with
fprintf('%s\n', TheCell{:});
Now fprintf the signature using a %Ns format where N is replaced with the maximum length . One way
fprintf('%*s\n', maxlen, signature );
thiss uses an advanced fprintf feature . you could instead use
fprintf(sprintf('%%%ds\\n', maxlen), signature )
