You can also use this method for lengthening arrows. Find the alum arrow that fits inside your short shaft and cut it about two inches longer than what is needed to lengthen the arrow. i.e. if the arrow is 2" too short, cut a 4" piece. Put the insert into the smaller shaft and hot glue the small shaft into the larger shaft. Then hot glue a 2" (in this case) piece of the larger shaft over the 2" of small shaft that's sticking out. Now your shaft is 2" longer, has a footed (strengthened) tip, and is a little heavier FOC. Kind of like a win, win, win.
As to the original question, I'd make my foot extend about an inch past the end of the insert. If they bend at the end, it's usually right at the end of the insert.