It doesn't have to be perfectly exact, but try to keep it as close as possible.
You didn't mention what dose and formulation of raltegravir you're taking, nor what other PEP drugs - and that matters, too. Raltegravir was originally a three-dose-a-day medication, until the manufacturer reformulated it. The liver clears it from the body relatively quickly, and that's why it's important to take it at close to the same time. If you wait too long, the body level can drop below where it's effective. Everyone's metabolism is different, and without actually monitoring the level of drug in your blood, you wouldn't be able to tell.
That said, the way they figure out the standard dosing is that it should work well in more than 95% of cases - including a bit of leeway for variations in liver chemistry, imperfect dosing, and so on. So don't worry, just do the best you can...