If I understand you correctly, the question is where should your female partner in mixed stand when you are returning serve?
A: She should stand about one foot away from the T, facing opposing net player and calling the service line.
The instant your return makes it past the net player, she should sprint like her hair is on fire toward the net. Once she reaches an appropriate position (depending on what you two decide that is), she should split step. Then every time opponents are hitting after that, she should split.
I actually think it is better to sprint toward the net than start there. It is a mental thing. I have a job to do and am more awake, you know? I have to watch the line, then watch the net player, then move forward. I also like having her in the line of fire if the net person poaches. She might get a ball or two back by being in the way of the poach.
Warning: If the serve comes up the T but is still good, she should move over so as not to block your shot.
It is too easy to get flat-footed if she just parks herself at net as you are returning and throughout the point.
-- Random Error Generator, Version 4.0
-- Master Moonballer