yes, that am on cd16 was probably just leftover lh, metabolized earlier. it's a good question though. i think there are a couple ways to test/record opks, it's one of those ambiguous things. here's a couple ideas:
1. record the trend- in other words, if you are testing 2-3 times a day (which is a smart idea until you get that positive) chart the trend... you get a negative in the am and a positive in the afternoon- it's trending positive. the next day it's pos in the morning and pos in the afternoon, chart pos of course, the day after that, it's pos in the am and neg in the afternoon- it's trending negative so chart neg.
that's how i used to do it. here's how i do it now, and i prefer it this way-
2. test 2-3 times a day (between 11 am and 8 pm or so) until you get a pos. after that, test once a day around 2 pm (this is statistically the "best" time of the day for opks) and record the result. then i continue testing opks until i get crosshairs after they go neg, once/day, because i have seen too many instances when people get one positive opk, think they are o'ing, and they dont. that's when the confusion really kicks in.
and just as a side note- never use first morning urine for an opk. you will hear ladies say that works for them, but it is technically in inaccurate result, due to the way our kidneys filter urine while we sleep and the way our brains secrete lh- the combo doesnt work, so to keep from being frustrated, i'd suggest you just dont do it. (the exception is the one brand that instructs you to- that test actually tests something in addition to lh as well, and i have no experience with that brand so i cant say much about it.)