IMHO, pre-ovy sheds are almost useless. Ive had "pre-ovy" sheds anywhere from 7-60 days from their ovulation. Unless youre specific female has a reliable pre ovulation shed every year, you really shouldnt try counting.

On the other hand, prelay sheds are much more accurate. On average, they lay around 30 days after their shed. Of course there is the odd female that lays on day 25, and there are a few stories of females making their keepers wait 45+ days before they drop their eggs, but overall it is pretty consistent.