OPK with IUI

Hi ladies,

My first IUI will be at the end of next month and my fertility doctor wants me to do an opk (with first morning urine) and call when I get a positive to then go in that day for an ultrasound to check follicles.

I’m using the Clearblue advanced. Should I be calling my doctor when I get the “high” positive or the “peak” positive?

I’ve read a lot of people haven’t used an opk prior to IUI, but this is what my doctor has suggested. Anyone been through the same situation and have any suggestions/input? Thanks in advance!