Its been many years since I last did this (1992 if I recall correctly)

I have a 125 litre (3.5cu ft) capacity mixer

If my failing memory serves me correctly it is ok to just over half fill it

I should be able to put in

60 litres of ballast (6 x10 litre buckets)

10 litres of cement

approx 10 litres of water

20 loads would give me about a cubic metre

Does this sound about right?