Except that I was wrong. Serves me right for doing arithmetic as I ran out of the house.
1/2 ounce of 24 + 1/2 ounce of 24 + 1/2 ounce of 12 = 1 1/2 ounces of 20mg
If you use HALF of the 24 mg (1/2 ounce of that plus the 1/2 ounce of 12 you have) that will give you 18mg
1/2 ounce of 24 + 1/2 ounce of 24 + 1/2 ounce of 12 = 1 1/2 ounces of 20mg
If you use HALF of the 24 mg (1/2 ounce of that plus the 1/2 ounce of 12 you have) that will give you 18mg