opk timing help

Hey guys, just wondered how important taking OPKs at the same time of day is?? I'm on CD 8 so not expecting a positive just yet but decided to test from the day my period finished this month as I'm struggling to get a positive on the tests so far. yesterday I tested at 2.30pm but today I'm going out for lunch with a friend, is it better to do it before I go about 12.30/1 or when I get back later in the afternoon. or does it really not matter and I'm over thinking!! thanks for any input x