You will normally have some idea of what a competitive pace is if you've done your homework. One thing to remember is not to spend yime watching the timer too often!
What I often did was go out and for a few laps just warm the bike and tyres up, then I'd run off a few quick laps to qualify and then I'd slow down again, check the times and, then put a few more in the bag for safety if needed.
They're a useful tool - not a panacea. And I don't need one to tell me when I'm riding like a dick.
