Weight changes swing tempo, contact feel and how easily the cue stops after impact. It does not create power or skill by itself. The useful range is the one that lets you repeat the same stroke without tightening your wrist.
Test it consistently
Compare cues of similar length on the same straight stop shot. A very light cue may invite a sharp jab; a heavy one can feel late and blunt fine control. Judge ten repeatable shots, not one impressive result.
Weight only makes sense alongside tip diameter, balance and shaft stiffness. Break cues serve a different job from playing cues. Ask for measured weight, removable weight bolts and any modifications on a used cue.
Choose the heaviest cue you can steer comfortably, then allow several sessions for adaptation after changing weight.