I shudder when I hear a vet give a day on cl's or follicles. The size is dependent on, feed, management, genetics, the cows status within the herd(if feed is not constant for all animals) etc,. What you need to hear is simply that she has a fairly developed cl. If time is a factor, lute is your friend. Otherwise start watching in a few days and watch for two weeks.
I tell young guys in my business(I AI for a living) that if they get a vet that calls a cl or follicle that they'd better give the cow a shot and make it happen on the scheduled day or get ready to lose the account. Example: vet calls cow heat plus three, meaning that he thinks that judging from the size of the follicle the cow will be in heat in three days. Cow is set to cycle in five days with a huge follicle instead of three. Customer takes vet at his word and waits for breeder to find the cow in heat on day three...like the vet said she would be. Breeder gets fired because he's missing heats that didn't happen yet.