PEP is very effective when it is started early within 72 hrs from the time of exposure.
It's suggested that PEP drugs can affect kidneys and the liver temporarily. So, it's advisable to keep a follow up with your doctor. You must stay hydrated and not miss your dose, in case if you do by chance, don't double up your dosage.
It is hard to quantify the accuracy of PEP with a number against it. However, it certainly very effective when started early. Also, absence of ejaculation does minimise the risk to an extend.
You must also, focus on the fact that if your partner is HIV negative, there is nothing to worry about. Although, I understand that it is easier said than done, if you could somehow find out about your partner's HIV status?