Measuring the gap between a ball and the rail with another ball is illegal. Wouldn't a cue with scales be a violation of the spirit of that rule?
I always wondered why cues never came with scales. I just figured it must be against the rules.
I don't think the WPA "official rules of pool" prohibit use of the cue or the CB (in hand) as a gap-measuring ruler. CSI/BCAPL rules do prohibit this:
1-3.1g - You may only use your vision to judge whether the cue ball or an object ball would fit through a gap, or to judge what ball the cue ball would contact first. You may not use any ball, cue, rack, chalk cube, or any other equipment or other part of your body as a width-measuring device.
Neither WPA or CSI prohibit using the cue to help aim/visualize shots. Here's the pertinent CSI rule:
1-3.1f - You may use your cue, held in your hand or not, to help align a shot.