Just to be safe id start the countdown once she sheds. Since you can't say for sure if she ovulated yet. I'd start checking her a few times a day just to make sure. But laying inverted isn't enough to say she is or isn't gravid. Once they are close, laying inverted is a good sign. But another thing to look for is being all twisted up like a pretzel. That means she's moving the eggs into place. Just because she locked with a male doesn't guarantee eggs.
Where is she laying in the tub? Has she been cool seeking/laying on the cool side or wrapping the water bowl? Those are signs of building follicles. Once she does ovulate she will move from the cool side to the hot side. That's when you may notice some laying inverted.
There is also a pretty decent color change. They lighten up a lot a kind of glow. You can also see them starting to look kind of lumpy. Then once they ovulate they move to the hot side, and color goes back to normal.
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