It would be very difficult to do without a bow scale. If you still have your manufacturer's literature it should tell you what one turn of the screw normally is. On my Mathews it is around 2.5 pounds.

One suggestion before you start turning is to take some paint or fingernail polish and make a series of dots on the limb bolts and the limbs that are lined up. That way when you make your adjustments you will be able to make one full turn (or half turn) on both limbs and not mess up the timing.