When to schedule an IUI using CBFM?

So this is kinda a unique situation for me, usually I have no problem conceiving quickly naturally, but this time we are opting for an IUI without chems using only fertility monitoring (CBFM and basal body temp).

Background: traditional surrogate for the adoptive parents of my second child attempting a biological sibling using adoptive father as donor. Family lives in DC I'm in FL so samples will have to be shipped or he will fly prior to have samples fresh washed.

I wanted to go a more holistic approach initially because I have no trouble concieving. Found a midwife who performs the procedure in office, however when trying to narrow down the window for insem. I'm not sure if I should stick to the peak days from the monitor or start on the last high day before peak day? I will have a "test run" for logistics purposes for future cycles but if I could help my chances of success on that test run then we are all for it.

has anyone had any success with a smaller window (2days max) ?