I don't even bother trying to calculate dates etc. If you get it wrong the problem comes when your wife applies for ILR and she hasn't completed the full five years. They seem to be very strict about that and I have read about a couple of instances where ILR has been refused over a day or two.
My wife's first FLR runs out in the 19th April this year. I will courier on the 16th or 17th to make sure that the application is in before her current FLR expires. Just as I did with her first.