The cleat should be roughly directly under the ball of your foot.
I get numb feet sometimes also; I have had this problem off and on for a while. Different shoes (which were wider) helped, and I realized that I needed to move my toes around as I rode. Backing off on the pedal pressure and spinning more helped too. I have chalked it up to poor circulation in my extremeties, as occasionally my hands start going numb just sitting with my elbows on my desk at work. It hasn't been bad enough to keep me away from 4-5 hour rides, so I don't think much about it anymore.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter