NCAA 7-11p:
A.R. 3—On a tag play between home plate and first base, a batter-runner may retreat toward home plate to evade a tag, but shall be declared out after touching or passing home plate, or leaving the base line. The ball remains live.
Therefore, a runner can back up to just before he would touch home plate.
newump,
You can
download the NCAA rules for free. Then, in this case, a simple search on "retreat" brings up the answer.
Some advice: When an ostensible umpire tries to rationalize a ruling based on "travesty of the game", and it doesn't involve an intervening pitch between the runner's advance to the next base and a subsequent return to a previous base, treat all of his comments as suspect.