I ride fixed rather than singlespeed. Chain tension is less critical on singlespeed. As hardboiled718 said, as long as the chain can't jump off, it's ok on a single speed. For comparison, think how much slack there is on a derailleur system, with that slack taken up only by a fairly weak spring in the derailleur mechanism.

What I do is stand the bike upside down, gently finger tighten the two wheel nuts, and pull the wheel back until the chain is tight enough. Then I nip it up a bit tighter on each side alternately. I always position the spanner so that as I turn it, the handle is moving towards the back of the bike, which, if anything, is applying a force in the direction of more tension on the chain. Whether this makes a difference is moot, but it works for me.

If your chain is too tight, it will result in friction and wear, and make your bike harder to pedal. If it is too loose, the chain may jump off. For a single speed, there is quite a lot of safe leeway between these two extremes.