Follow the directions on the package of the OPK. Yes, the Clearblues say to use FMU but most brands are meant to be used after lunch since that's when the hormone reaches a peak in your urine. many people seem to get best results after lunch even with the Clearblues - but since the Clearblues are detecting changes in your hormones and not only LH alone they are supposed to be able to be used with FMU instead.