If you cant do it at noonish then later is better, to what i have read.....
I also peed in a cup to do mine...noon wasn't a reasonable time for me...I'm a teacher so I tried mornings and evenings...
To be safe, I'd start on day 10 (day one being the first day of af)...you might want to buy more because sometimes testing once a day isn't enough...I've seen people test in the a.m. and get a 'negative', test in the afternoon and get a positive, and test at night and get a negative...This is why I personally don't like opks. I did much better with the basal body temping and using the fertility friend website....Good luck!
Oh yea, you want to test at about noon time, everyday at the same time...Remember to hold your pee for a good 4hours before testing.....
Just to be sure, I would start a few days after Af ends. Yes, you might need more then 7. Remeber that your line has to be as dark or darker then the control line. I found it was best to pee in a cup and then dip the test in that. Good luck!