To help prevent mold I would keep the humidity level below 60%. enter link description here As for your runny nose, as people have commented that it could be a mold allergy. I would suggest getting your basement tested for mold. I've seen test kits at home improvement stores, but I've not used them before so I can't attest to their effectiveness. Another thing you might want to do is clean out your dehumidifier. They tend to get pretty nasty over time and might a source of your mold. Generally, they need to be cleaned out once a year and in some case more often.
Another trick that worked for me to reduce humidity in my basement is to put plastic over the basement windows. In the Northern part of the USA, during winter time it's common to find these plastic window kits. The plastic sheets go over your windows with double sided taped. Once attached, you tighten them up with a hair dryer. The idea is to prevent drafts that get past poorly sealed windows. In the basement I've left them on all year round, and I've noticed a 10% reduction in humidity. That might save you some energy from having the dehumidifier as much.