Firstly, make sure your chainring and cog are in-line. Then, ensure that the wheel is in-line with the frame. After that, you can make it as loose as possible to where you still can't get the chain off the ring. Generally, that means that at the mid-point between the ring and cog you can move the chain up and down between an inch or two (total distance, not in either direction)
If you're coming from a road or MTB background, the slack track chain will always look like there's something wrong, but it is, in fact, the chain that is drawn taut between the ring and the cog that has the problem.