I've never tried to shave my head with a straight, since I stopped shaving my head before I started using a straight. However, I would REALLY discourage trying to shave the back of your head with mirrors unless you've done a lot of work with multiple mirrors and you're very comfortable with it.
It's very easy to feel the stubble on skin that is stretched as tight as the average scalp.
What I always did was soap up, and then I would keep my left hand flat on my head and rub back and forth over the area i was shaving.
This does three things:
1. It continually redistributes the soap, which is good because lubrication is really important on sensitive skin.
2. It really gives you a good idea of where everything is, and how it's all shaped. This is a very rear-brain activity, and that's where the information collected from the feeling in your hands goes.
3. It makes it really easy to find missed spots.
Also, like several other people have suggested, I'd to one thing at a time. IIRC, the guy who went 3 months before his first nick, did it largely by starting slow. I'd get comfortable with a straight first, and then try putting it on your melon. Learning to do both at once seems like a good way to lose an ear.