Slightly rubbery is probably slightly overcooked, but I'm sure they were more than edible!
I've overcooked my share of scallops, and I've finally found my own foolproof method.
I have some flat, wide skewers made of stainless steel that I put the scallops on so the don't swivel on me. I designate one of the scallops a "chef's" scallop and make a single cut from the center of the scallop outward, kinda like you were making a single cut in a pie.
As the scallops cook, the cut opens up slightly and you can see the scallop turning colors from a translucent hue (uncooked) to a milky-white (cooked). Once the scallop has barely turned completely white, I pull those suckers off and plate