Damien, you said he needs 12.8 oz of oil. Sounds like what he did. 1.6 cups is about 12.8 oz. 8 + 4 + .1 oz = about 12.8 +- a bit.
You people should use the metric system to mix fuel, way easier. 20:1 is 1000 divided by 20 = 50 Convert that to ml is 50 ml per litre. 30:1 is 1000 divided by 30 = 33.3 ml per liter. A litre is 1000 ml.