From what concrete I have ordered they ask "how many sacks do you want"  They mean how many sack of concrete pre yard of concrete.  More sack, stronger concrete, I think.  Min. I have ever gotten was 5 sacks.

Do you have a float?  Local company here will lend you one if you need one.

I would limit your strikes over the concrete to 2 if possible, no more than 3.  If you go over to many times you take out to much concrete and it will be low in the middle and water will stand.