In reply to featuresforfeet:
I'm 6ft though I don't think your height should be limiting.
I set the length by sitting on the MT so it is engaged then putting the shunt above with a small gap and connecting to it via sling and maillon. Set the required length of sling extender with a knot. That way I know that if the shunt slides it will not interact with the MT by the time the MT is fully engaged. I'm probably making it sound more complicated than it is.
I'm not sure what you mean by something pushing up on the shunt. When you are climbing, the MT is dragging downwards as low as it will go while the shunt is held chest height with the chest harness. There is more gap between the devices when climbing than when sitting on the rope. They never touch each other.
I don't like tying backup knots as it is faff. Also, I mainly climb sea cliffs where I abseil down the line I climb up. When I'm finished I don't like the possibility that I pull up my rope with lots of knots that could get caught in cracks etc. I have also read that a high factor fall of a shunt onto a knot can cause the shunt to deform, potentially enough for the rope to come out. If using knots as your backup, make them fairly regular. I prefer the MT as backup.
You can have the devices either way around (I have), I just prefer toothed device below. Just bare in mind the risks and limitations of both in whichever configuration you choose.
In the end this is just the way I do it, and there are many ways people achieve the same thing. It may not suit everyone. You just have to be comfortable with whatever method you use, otherwise you won't climb freely and I'd argue that if you don't trust your fall protection you might end up trying to grab the rope or device, or otherwise hesitating in a way that could be dangerous.
Take from that what you will. Be safe and have fun!