I heard so many bad things about the accuracy of the ttl prism that I went with the standard prism. I take an incident meter reading and adjust from there. It's worked well for me. If you want to be exact about it a spot meter would be the way to go.
All in all it's a great camera. Make sure you get the mirror lock-up version or slow shutter speeds will cause your images to suffer from vibration. Also, if you're shooting with flash you're stuck at 1/30 or slower sync. There are available leaf shutter lenses if this is a problem. Try to get the newer "67" model verses the "6x7" or get a 67II. If you give this camera what it wants it will make beautiful, sharp images for you. And the Pentax lenses are sharp, cheap and plentiful. I sold my more valuable Mamiya 7 because it just made nicer images for me.