How to keep horizon flat in 2 point perspective?

Does anyone know if there is a way to "lock" in the horizon line when moving the points on the 2 point perspective tool?  I can move the points fine, but the horizon line does not stay flat as I move the points around, which it great if I'm doing a dutch angle image, but not so much for a normal flat image. I can of course eyeball it, but that can be tricky with all the other perspective lines, thanks optical illusions!  Any suggestions?