Is there a point that is too far? Balk?

[Publius]

You'd be correct. "Directly" is interpreted by umpires of driving-age players to mean, "more toward the base being thrown to than toward the adjacent base."

If you teach your pitchers to step on the imaginary line, or on the first base side of it, competent umpires will allow it as a legal move. You'll still get balked sometimes, as:

A: you won't always have competent umpires; and

B: your pitcher will sometimes miss the line and land on the home plate side. In the ensuing discussion, you, the catcher, and he will all claim his foot was on the imaginary line.