I added 0.4 oz of oil to 1/2 gallon of gas too much to mixture. How can I dilute the mixture without starting over?

You should mix 8 Oz. of two cycle oil to one gallon of gas. I put the oil in first and then the gas and it will be mixed and ready to go.

Mixing
RatioGAS:OIL UNITED STATESOIL per 1 U.S. GALLON Gasoline
(128 oz.) *INTERNATIONALOIL per 1 IMPERIAL GALLON Gasoline
(160 oz.) *INTERNATIONALOIL per 1 LITER Gasoline
(metric) 32:1 4 oz. 5 oz. 31.25 mL 40:1 3.2 oz. 4 oz. 25 mL 50:1 2.6 oz. 3.2 oz. 20 mL

Two cycle engines run on a mixture of gasoline and oil. Each engine type is designed to run on a specific gas-to-oil mix ratio.
IMPORTANT: Use only oil that is labeled for use in "Two-cycle" or "Two-stroke" engines. Home Depot and other stores now have "premixed fuel" of gas and 2 cycle oil.
To mix two-cycle fuel, start by putting correct amount of oil in gas can. Then slowly add correct amount of gasoline. Do not try to mix gas and oil directly in gas tank of equipment.

IMPORTANT: Do not use any fuel that is over 30 days old. Do not mix more fuel than you are likely to use in a month. Old fuel can gum carburetors and prevent starting. Always use fresh fuel.

The chart below shows correct amount of oil per gallon that each mixing ratio requires. Expect that yours will be between 32 gas to1 oil and 50 to 1. 32:1 and 50:1 are most popular.
Ratio GAS:OIL UNITED STATES OIL per 1 U.S. GALLON Gasoline
(128 oz.) *INTERNATIONAL OIL per 1 IMPERIAL GALLON Gasoline
(160 oz.) *INTERNATIONAL OIL per 1 LITER Gasoline
(metric) 32:1 4 oz. 5 oz. 31.25 mL 40:1 3.2 oz. 4 oz. 25 mL 50:1 2.6 oz. 3.2 oz. 20 mL

2.56 oz of oil per gallon is the exact amount. Dump 3 oz. of oil in a gallon gas can and fill it up. Then mark the can so you don't mix it up with lawn mower gas.

Perhaps you should have put more gas or less
Chemtool B-12 in.Would suggest a top up if you
can get to pump.Should dilute Chemtool enough to run
properly.

for a 40/1 mixture use 3 ounces of oil ( 2 stroke) to a gallon of gas for a 50/1 ( which is what I recommend ) its 2.5 ounces to each gallon of gas, if you find that the machine is smoking a little too much,, back up on the oil just a hair,, but I don't think you'll need to

try adding a bottle of heet water remover into the gas tank , this will help dilute the fuel injector cleaner.

Ratios range by machine and manufacturer from about 32:1 up to 50:1. 40:1 is most common.

32:1 4oz. oil/US gallon gas
40:1 3.2 oz oil/US gallon gas
50:1 2.56 oz oil/US gallon gas
Here is a chart with many different ratios:
http://www.rctek.com/pdf/fuel_to_oil_ratio_chart.pdf

It is 2.6 fl oz of oil per 1 gal of gasoline.

3.2 oz oil in one US gallon yields 40:1 mixture.
2.6 oz oil in one US gallon yields 50:1 mixture.