A good LP should be 12 days or longer according to a book I have.

My last cycle was 30 days but I O-ed on CD20 giving me a LP of 10 days which is a bit short.

The cycle before that was 36 days and I O-ed on CD23 which is a LP of 13 days.

So it keeps changing for me!

You could expect your cycle to be shorter having O-ed on CD12, but I don't think this is always the case.

With my last cycle I O-ed 3 days earlier than the one before so I expected my entire cycle to be 3 days shorter - but in fact it was 6 days shorter. So I have no idea what it's doing!!!