New to these - not a clue what i am aiming for. Ride flats on my mtb
The middle?
I've only had mine just over a month. I followed the instructions in the box for initial fit, but then had bike fit and they were moved as far forward as would go and set fairly square (to my eye anyway). Seem fine so far with no aches or pains.
dead helpful
Set them so the ball of your big toe is slightly in front of the axle of the pedal, 10mm or so. As for angle see how you walk, toe in, toe out or pretty straight and replcate that with your cleats. Then play with them for the next 20 years till you find what's right .. 
What direction do your feet point in? I'm a ten to two man. So I set my cleats angled inwards about 10 degrees and in slightly from the Centre to avoid heel strike on the chainstays. Ball of foot is above the pedal, but find this isn't sensitive.
If your feet point inwards, do the reverse. Site on a table and dangle your feet out beneath you.
It depends on so much. With super stiff dancing shoes you can have the cleat well forward, but you can't sort with advice on the net.
Have a play about, take a key with you to adjust things mid ride but give it time between each adjustment to allow your feet/legs/body to adjust.
Think about where you would like the axle of the pedal to be for maximum efficiency.
Sit on a desk or your kitchen table or somert. Let your legs hand over loose, free. look down and see how your feet are positioned, draw an imaginary straight line through the ball off your foot and thats how your cleat should be mounted on your shoe. If you have a 5 degree or so amount of float in your cleat/pedal all should be ok. I think I have explained that, well semi understandable.