Jim (Creative Engineering) is about right- about 1/2 liter.
Here are the numbers:
1 liter of gas/oil mix at 20:1 (that is, not one liter of gas plus oil, but one liter total of gas and oil) contains:
.04762 liters of oil (47.62ml)
.95238 liters of gas (952.38 ml)
If you have the same amount of oil, 47.62 ml, the how much gas do you add?
Multiply .04762 liters by 30 = 1.4286 liters.
Subtract how much gas there was from how much you need:
1.4286 - .95238 = .47622 liters of gas to add.
It will vary a bit depending on rounding and how many decimals you carry it to.
So, about 1/2 liter, but a little under. If you had 1 liter of gas plus oil at 20:1 (50 ml), you would add exactly 1/2 liter.