A good rule of thumb of 7% for older premix twostrokes.
The following is extracted from the Villiers forum, and is applicable to most twostrokes built before the mid-1960s.

As far as premix goes the 16:1 is still correct for running in a new engine and you can then cut down to 20:1 thereafter. Villiers began to recommend 20:1 in the early 1950's when self mixing two-stroke oil first became available. This is appropriate for both the engines you have . You will be told you can cut down the oil a lot if you use a modern synthetic oil. Do not do so. Synthetic oils are meant for modern two-strokes designed to modern standards. The older design of Villiers engines means they need a lot more oil. They are the last of the early design of two-stroke, for best results stick to 20:1. Because they are a slow revving engine compared to later two-strokes a two-stroke oil suitable for lawnmowers, chainsaws, etc, is fine for them. It will smoke when it is cold, it will smoke if you only travel short distances but it will stop smoking when it is hot and stop smoking if you ride it any distance. If a pre-mix two-stroke is smoky all the time its the riders fault not the bike. It is not being ridden far enough or long enough to keep clean. For years I rode two-strokes to work and they would get smoky in all the stop/start traffic. When it got too bad I would go for a fast ride on the local freeway which would clear the exhaust of the oily build up. The bike would then run sweetly for another month or so and I would then repeat the exercise.

If you had an old bike with an ordinary four-stroke engine and someone told you it would run better if you only half filled it with oil you'd think they were nuts. Cutting down on the oil in premix is exactly the same thing and just as foolish.

Do not bother with premium grade fuel. Again, because these engines were designed in the days of low octane fuel there is no point in using anything up market. Standard unleaded fuel of today is far better than anything available when they were new. Also, they are unable to take advantage of anything better so it is just a waste of money and you won't get any better performance. Always use unleaded. Leaded fuels can quickly foul up the spark plugs on Villiers engines and be a real nuisance. The introduction of unleaded fuel was a real bonus for two-stroke riders as it got rid of a lot of spark plug problems.

by 33d6

This page gives very basic information as a general guide.
dansmc.com/pre_mix_ratios.pdf

rctek.com Fuel to Oil Ratio Chart
rctek.com/pdf/fuel_to_oil_ratio_chart.pdf

Wikipedea on 2 stroke oil
en.wikipedia.org/wiki/Two-stroke_oil

Two-Cycle Premix Ratio Calculator
http://www.scooterhelp.com/genmaintain/mix.calc.html
Fuel - Oil Ratio Calculator
itslookingup.com/moped/oilratio.htm Oil Ratio Mixing Ratios (404)

Villiers Two-Strokes
http://www.tanygraig.force9.co.uk/John/vil2strokes.htm

Sachs 301 fuel mixing
spartamet.org/english/e_brandstof.html

See also:
Troubleshooting
Fuels