You really cannot tighten a fixed cog properly by hand or by rotafixing... you do of course torque the hell out of it when you install it but after that first ride around the block it will be tighter than it was when you started riding as the torque you can apply by pedalling far exceeds that which you can apply with your meager arms.

This is when you go back and check the lock ring which should have also been installed with a high degree of torque and bet it will tighten up just a little more. After that you should be golden... make sure you lube the threads to prevent corrosion and to allow the cog and lockring to seat.

I am just a little guy... after installing rear wheels and torquing the nuts down to spec I know a lot of bigger guys who cannot undo them. The rear cog has an even higher torque spec and anyone who has tried to remove a well ridden in cog / lockring knows how tight they can be.

Biggest issue I see with fg bikes that come into the shop is that the initial installation of the cog and lockring was done improperly... in many cases the bikes came this way from the shop that sold them because the mechs do not understand the principles of proper torque for fixed drives.