I also had trouble with this. When I fully shutdown my computer and restarted it the RAM was down to 70%. But an hour later it was back up to 98%. I just plan on fully shutting down my computer right before I take the exam. Hopefully that works!
I have done several tests through ProctorU with my macbook air (the LSAT and others for classes I have taken) and it has always run smoothly! Just make sure you have enough free RAM to meet ProctorU's requirements