if FR directions are telling you to use fmu, then follow the directions. The opks I used directed me NOT to use fmu, and poas around mid dayish...you will always get a faint line since you have that hormone in your system...Keep watching it, like you mentioned it is getting darker, so that it a good thing!

BD when it is close to or daker then the control line.
I personally don't like opks much at all. I love my clear blue fertility monitor though...(even though it didn't show a peak for me this first time around).