What is the DC for using concentration in Kiteticist Blast?

[Dante Doom]

Since the DC is spell level dependent how do i calculate the DC to use concentrate to avoid an AOO ?

[Sammy T]

15 + (Kineticist Level/2) x 2 = Cast Defensively

L9 Kineticist example

15 + (9/2=4.5, round down 4) x 2 = DC 23

[Dante Doom]

Thanks!