If you had 816 billion cells/2L, that is 0.408 billion cells/ml (or 408 million cells/ml) of which you took 1 ml, so you had 408 million cells/ml, which you diluted 10%, so you had 40.8 million cells/ml, then diluted that 20%, so you have 8.16 million cells/ml, then you diluted 50%, so you have 408 million cells/ml. Or yes, that is a 100 dilution factor, so it is 408 million cells/ml times 1%, which is 4.08 million cells/ml.

I don't know anything about the counting, but that should be what you ended up with.