they always say when the line is as dark or darker..... but personally with my experience i thinks its whats dark for you, if you always have no line and then have a fearly dark line and then no line id say that was your surge for sure!!!! also maybe you just missed the main surge say if it happened soon after you tested last so maybe it was on its way back down. everyone is different and opks are not that easy to read. i always have no line and then an almost positive at ovulation and then a faint line that fades back to nothing. as long as you bd well i wouldnt worry. opks can be stressful and confusing but you had other signs to say that was your surge anyway.