Hi Jo,
The dreaded opk dilemma! Many ob's will say to use your opk's first thing in the morning. The reason why the manufacturers say not to use fmu is this.
FMU is concentrated. You haven't had anything by mouth for 8 hours give or take and the urine then is strong or concentrated. LH hormone is always present in human's urine. So it makes sense that a concentrated sample will show a build up of the LH hormone that is always present and may mimic a surge. Are you with me?
However MANY women do not get positive opk's. Possibly because they are not 'catching the surge'. I personally only ever have 'caught the surge' when using fmu. I then test later in the day and get an almost but not quite positive.
I would go with what you are doing. If you don't show a positive at any other time of the month using fmu then what you are seeing I would say is a surge.
Also, it is widely thought that once you see that first positive most women will surge within 24 hours from the first positive. It is possible for women to continue to get positive tests for a few days. Depending on their levels of LH after the surge. So it's important to make love from the first positive for a few days in a row. Latest research has said that unless your partner has a sperm count issue that daily intercourse gives you the best chance of conception.

So, from when you first see a positive start to make love each day for a few days. You won't know exactly when you ovulated but around when you get your positive you will ovulate. So you go girl!!!! Wishing you lots of conception vibes...