IUI question

I'm tracking my cycle myself now. I'm using frozen sperm, which I know isn't as viable as fresh. When should notify SGF of when we should do the IUI? On the day of which I get a positive opk? Would that be too late? Last time, I did the IUI when was still getting a high fertility rather than a peak fertility on my opk and the IUI didn't take. I did the IUI on the 4th with high fertility, but received a peak on the 9th. Any advice is much appreciated as I'm hoping March is my month :)