I don't know the proportions of mixer to petrol...

Question

I am using a friends McCulloch Blower Vacuum 243 whilst he is on holiday, but have used all his pre-mixed solution. I don't know what the proportion of mixer oil to petrol is. The mixer bottle states fill with petrol to 20:1, 25:1, 40:1 level - which is the correct level for the 243?Can someone help me please.