Teaching Elder,
In Federation rules, batters may not permit the pitch to hit them.
There is considerable subjectivity in deciding whether a batter "permits" a pitch to hit him or simply gets hit by a pitch.
I endeavor to give batters a fair opportunity to take a normal load and read the pitch before attempting evasive action. On breaking balls, I give them a chance to stay in long enough to make sure it won't come back to the zone.
The higher the velocity, the less I expect them to react. I keep them in the box when the apparent evasive action is a deliberately ineffective pretext and when they take no evasive action despite having ample time to get out of the way.