Neil, your math is correct and I too have been using the same 1% rule or 200 ml for each 20 litre Gerry can.  With 150 hrs on the plane now, maybe I should cut back to 160 ml as well.
Jeff

Gents,

                 Further to the recent discussion you all seem to put one ounce in your US gallon?  Your US gallon is only 4 litres in Aust terms.  Apparently according to Dr Google there are 128 ounces in a US gallon.  That according to my maths is 0.8%.  I have been consistently been putting in 1% .  That is 200ml in 20 litres of fuel.  Now it appears that I am somewhat "lavish" with the 2 stroke oil.  If that is the case I now know why I continually "make" oil in the sump.  0.8% now means that I can cut back to 160ml per 20 litres?

Correct??   Neil.