Which directivity to specify ?

It really depends on your loudspeakers. In some cases, if you want to adjust the slope of the target curve, you could change the directivity at low or high frequencies set in the form.

For reference, here are the values used for calculation :

High frequencies, Omni/Dipole/Standard/Horn = 2/5/8/12

Low frequencies, Omni/Dipole = 2/5