[Andy]

First of all, make sure her foot is down in front of the #2 postition player at the contact of the ball. I have seen a lot of setters that can time it out just right so that their foot hits the court just as or just after the serve is contacted.

As for how strict to call it - here is my philosophy.

Ask yourself this;
What is the purpose of the setter moving?
To get to an advantageous position somewhere in the middle of the court to receive a pass and run the offense.

If she is allowed to leave early and overlap to get to that advantageous position, her team has gained an advantage by violating a rule. You are a basketball official - apply advantage/disadvantage to this situation. If it is minimal, let it go or maybe just say to her...:"don't go too early" That lets her know that you are watching her and hopefully slows her down a bit. If she is gaining an advantage in getting to her spot, call the illegal alignment.