in the old days , with soft rubber bushings , you would tighten the nut until the rubber expanded to the diamwter of the washer-----with urethane , i just tighten them up until you can't rotate the bushing with your fingers
i put urethane bushings on my old '47 sedan and they promptly tore the brackets off the lower control arms---i went back to rubber and have had no issues in 13k miles---i order the orig rubber for any installation from the mustang resto shops
doctordesoto