Best of luck to you on your deployment.
To be on the safe, less problem side, have your wife get a quart of diesel additive like Power Service. It is good, relatively cheap insurance for problem avoidance in the winter. I intend to start treating all of my summer fuel and what's in the tractor's tank starting in October. Because I expect the tractor's use to go down in the winter, and because I have about a 20 gal tank on the tractor, I will start early in the treatment process. I am not sure I will even use the full tank of diesel in the winter, so I decided it is better to be safe than sorrry.
At least, for the Power Service brand diesel additive, you can put in a bit more than what is called for, and there are no negative consequences for mis-measuring.
Please let your wife know that if she does have any tractor problems or questions, don't hesitate to contact TBN. We will all do our best to help her out.
Just saw your second note. No harm in putting in the additive and topping off the tank now, vs later.
Regards, Roy