You need to add 3 ounces of oil.
Right now you have 20% Nitro, 16% oil and 64% Methanol.
After adding 3 ounces you will have 19.5% Nitro, 18% oil and 62.5% Methanol.
If you want to play with numbers a bit more go to
http://www.nitrorc.com/fuelws/
or if you want a calculator that would help you in making your own fuel from scratch then go to http://www.angelfire.com/in3/aeronuts/fuelcalc2.html

Hope this helps