male/female percentages in straight run

[Rouen Mom]

Is there a rule of thumb about how many ducklings I should order in a straight run batch if I want to end up with a certain number of females? I know that this is not an exact science, but is there a generally accepted method?
I want to order a very small (10 or 12) straight run order, but I need at least 4 female, Do I need to order more than 10?

There's websites where you can order either male or female ducklings based on your preference. I have never ordered ducklings personally (much too impatient to wait days for duckies to travel in mail, want duckies right away) but I've seen other people send for them and they were all very happy.