Why don't just adhere to the manufacturer? Airplane manufacturer of the types that I fly says: "The captain has the sole responsibility for the decision to reject the takeoff. If the decision is to reject the takeoff, the captain must clearly announce 'REJECT', immediately start the rejected takeoff maneuver, and assume control of the airplane". Doesn't mention the F/O calling out to reject. He or she would reject only following a pilot incapacitation, IE 80 knots call....
Forgot to add, by following the manufacturer recommendations, your @ss and the airline has been covered in the event of a mishap....
But then again, what do I know....