In calculating the distance covered by a kick returner, how is his starting location determined? I believe that it should be the location of his rearmost foot at the time he catches the ball. I have seen returns described as "100 yards" when the receiver had one foot on the goal line and one foot one yard deep in the end zone. Obviously, however, if he had one foot just in front of the backline of the end zone and one foot over that line, he would be ruled to be out of bounds. His position in that instance would certainly be determined by his back foot. I would think that the same logic would apply in determining his position anywhere on the field.
For college, this statistic is explained in the NCAA Football Statisticians' Manual. Kick return statistics are defined in Section 8. From the 2016 edition:
Article 2. Returns are measured from the point where the returning player first gained or lost possession of the ball (using the back foot as a starting point) to the point where the ball is declared dead or is lost by a fumble. Note: A muff is an unsuccessful attempt to catch or recover a ball that is touched in the attempt. Exception: On kickoffs that go over the goal line, the return is measured from the goal line to the point where the ball is declared dead.
Normally, the back foot at the point of the catch is the starting location for the return measurement. If the ball is caught in the end zone, the goal line is the starting point for the measurement. As a result, 100 yards is the maximum length of a kick return in college.
The NFL treats this statistic slightly differently, described in the NFL Guide for Statisticians (PDF). First, they specify that the return does not start until the "impetus of the kickoff ends and he is able to initiate forward progress." Second, they do not have the exception for balls caught in the endzone, so it is possible to have kick returns longer than 100 yards. (Thanks, New-To-IT, for reminding me of this in the comments.)
From the 2016 NFL book:
If a kickoff is caught by a receiving team player, the spot of his possession is the point at which the impetus of the kickoff ends and he is able to initiate forward progress. This includes the end zone area. For example, if a player catches the kickoff at the 1, runs back to the -2, then advances, the spot of possession is the 1. If a player catches the kickoff at the 1, but retreats to the -2 because the impetus of the kickoff requires him to do so, the spot of possession is the -2. (NOTE, “spot of possession” is defined differently here than in the playing rules.)
A kickoff return begins at the spot where a receiving team player first gains possession of the ball, and then attempts to advance the ball.
Ben has a good formal answer but the fact is this is all over the place unless you are talking about NCAA Division 1 or NFL which both have clear guidelines.
If you are talking about NAIA, high school, other college divisions, pee-wee, junior football, whatever... each organization either has its own rules or teams are at liberty to score how they feel.
Meaning you will see many colleges outside of the NCAA Division 1, still score kickoffs past 100 yards. Most high schools still score like this too - albeit most states have a "no return" rule if the returner's foot hits the goal line.
Now as a scorer for several high schools and colleges I was taught you look at the heel of the back foot and choose the "next yard up". Meaning if the back foot is 5 1/2' behind the goal line then that is a 101 yard kick return. So even if a guy has both heels 1/8" from the back line all he gets is 109 yards.