No day 12 should be fine.. Also best at the same time of the day. 2pm is apparently the best time, but I did mine between 12 and 1.. Not as simple as POAS unfortunately lol.. They are exxy too. You can get cheap ones on E bay, I did and they work.. I found temping and OPK's good.. Either one by themselves I found too confusing. I liked to see a pattern of positive OPK's and temp dips lol..