I normally use OPKS twice a day now as it's easy to miss your LH surge I think. 2-3pm is a good time if you're doing ti once, for me, I do do 10-11am then again 3-4pm. But it's probably different for everyone. WIth regards to the crosshairs on FF, I *think* you need at least 3 temps that are higher than the highest in your pre-ov range? or something similar lol. I'm on CD19 and don't have my crosshairs yet, but should get them tomorrow as long as my temps stay nice and high. 
Good luck, it's all a bit confusing to start but you settle into knowing what works best for you quite quickly 