Welcome to TCS
Feline gestation is approx. 63-69 days. Since you dont know a mate-date, it's kind of a guess, as you may know by reading some of the other threads. Even a vet's ultrasound is known to be off by a week.
Have you felt the kittens move yet? Kittens can usually be felt two weeks before delivery. The farther into the pregnancy the more pronounced the movements. Within the last week you can actually see the kittens mover without touching her.
Also within the last week her mammary glands will begin to fill. Do they feel swollen to you? Not the nipple, but the glands along her nipples.
It is common to see some discharge days before delivery, but if you notice the mucous plug expel ahead of time you can guess she will deliver within 24 hours. However some cats dont lose this until they are delivering.
Also, some cat stop eating before they deliver, but I have had fosters eat *while* they delivered, so it is hard to predict with that.
You mentioned that you set up a birthing box, I would recommend setting up a few of them. For the current foster we h ave now, I have set up an open-topped box, a covered box (with a flippable lid for checking on her if needed), her travel crate and also a pile of old sheets. You never know where they will prefer, but I hope this deters her from crawling under my dresser!
Also this past week I have confined her to my room with food, water and litter of course. She doesnt really seem to have an inkling of escaping, but I want to make sure she has the quiet she deserves when it is time.
Speaking of food, if you havent already, now is a good time to feed her kitten food. She can use all the extra calories. ESPECIALLY after she delivers.
BTW - what is her name??