I have a barometer in my shop. I've shot as high as 70% without problems, but you are in the range at that point. Generally anything above 60% without retarder or at least extra thinner is going to be a problem. When it gets on toward 80% (which it is a lot around here), you can hit it with retarder and extra thinner and cross your fingers. It might be OK. Maybe. Anything above that and it's just not worth it.