The chain shouldn't have tension on it, but should have a wee spot of slack even in the tight spots. The old trackies' trick is to set it as loose as possible, pick the bike up, give the cranks a good hearty spin, and shake and invert the bike to see if it comes off. If it comes off, move the rear wheel back 1mm, test again.

Also make sure that the rear cog is pointing straight at the chainring. This may not be where the rear wheel is centered between the chainstays.

Good luck

Mel