adf4360-8 Lext value

hi

i'm using the ADF4360-8,

need to know the recommended value of Lext if my output frequency is 320MHz.

i tried 56nH but the PLL locks on 288MHz. in parallel  to  Lext i have 470 Ohm resistor

i'm using REF clock of 40MHz with R=10,

will appreciate any help

thanks

eli