First, good luck with your breeding this year! I'd still pair them once every two weeks or so until you have a confirmed ovulation.
Has she started to lighten up any for you? Usually people will mistake building for an ovulation - but an ovulation will be rock hard. So - if you don't see any noticeable swelling, it's hard to say if she's building yet, but if she's glowing, you know you're on your way!