K&R C Programming Language: pg. 105

Extend entab and detab to accept the shorthand

entab -m +n

to mean tab stops every n columns, starting at column m.

entab replaces a number of spaces with a tab character and detab does the opposite. The question I have concerns the tab stops and entab. I figure that for detab it's pretty easy to determine the number of spaces needed to reach the next tab stop, so no worries there. With entab, replacing spaces with tabs is slightly more difficult since I cannot for sure know how large the tab character goes to its own tab stop (unless there is a way to know for sure).

Am I even thinking about this thing properly?