ADF5902 CLK1 setup

I have a ADF5902, I want to setup CLK1.   But it's Register 7 have follow description:

"The CLK1 value sets a divider for the VCO frequency calibration. Load the divider such that PFD frequency (fPFD)/CLK1 is less than or equal to 25 kHz.

For example, for fPFD = 50 MHz, set CLK1 = 2048 so that fPFD/CLK1 < 25 kHz. "

fPFD/CLK1 must less than  or equal to 25kHz ? 

If it must be less than  or equal 25kHz,  timer = CLK1 x CLK2/fPFD  is  not less than 40us.  The timer is  too long.