I'm getting too good at using these!
Unlike a HPT you don't want to use FMU because it concentrates the LH... what you look for in an OPK is the intensity of the line, not just a positive or negative like in a HPT
I get mine from www.saveontests.com
and they have the little strips that you dip in the urine... they are VERY cheap. I have found their OPK's and HPT's very reliable. I do not O regularly and really appreciate having 50 OPK's in my cupboard!!
Anyway, I test mid-day or right after I get home from work (most convenient) then when I very first start getting a line I take it twice a day. You can see that the line gets darker each time, then starts to fade over a couple of days... corresponds to the surge... anyway, you O about 36 -48 hrs after the start of the surge and 12-24 hours after the peak.... please double check this, I'm just going from memory and my experiences.
The instructions on the box say it's positive only if the test line is darker than the control line - I say baloney because since you don't take these every hour you might not test at the peak... that's why I suggest looking at the trend of the OPK getting darker then lighter again.
Anyway, I hope this helps.
Good luck and babydust +~+~+~+~+~+~+