The rule is to multiply the R12 fill spec by 0.9 then subtract 0.25 pounds from the result.
Example 40 oz. of R12 = 2.5 pounds X 0.9 = 2.25 pounds minus 0.25 = 2.0 pounds of R134a. Keep in mind those small cans are only 12 oz.
Additionally if you would like it to cool better put an auxiliary fan on the condenser then you can add a few more ounces of refrigerant. Keep the high side around 250 psi.