Most tonearms have some sort of calibration, very often on the counterweight. What would help us help you is both the cartridge and the arm models, or a picture. Ultimately tracking force should be set by ear, so if the calibration of the arm isn't perfect, its not really a big deal, since its just a place to start. Most cartridges have a range specified, and if you start something above the middle of the range, you will probably be fine unless your arm is terrible. (The thing that really destroys grooves is when the stylus looses contact with the walls of the groove and goes careening around smashing stuff, so low tracking force is worse than high. Assuming normal limts of course -- eight pounds is a terrible idea.)