I see some potential problems with rolling a cue off the top of a rail to check the straightness. How many times have you felt the strip that holds the felt on the top of the rail either above or below the the level of the rail or cusion. Any small imperfection of a few thousands will be magnafied over the legenth of the cue to where the tip is. I would think a flat surface with a raised rail of some sort would provide better results. At the shows I attend I see several buyers and sellers that test cues this way. I say do what works for you but I prefer to roll a cue on a table and look at the taper of the shaft and see if the gap from the table to the shaft changes as it is rolled. If you are looking for the most precise way of checking a cue you really need to put it in a lathe and use a dial indicator to check runout. Chris.