Can any one suggest, how to calculate the MOD and FRAC values for ADF4351 manually?

I'm able to get the MOD value for certain frequencies when i am using the below formula

MOD=PFD Frequency/Channel spacing*1000

But i'm unable to get the correct values if i'm using the above formula for certain frequencies like 3851MHz,1254MHz,2704MHz,....etc

Is there any other formula to calculate MOD and FRAC values manually?

Take a look at

RF SYNTHESIZER - A WORKED EXAMPLEon page 21 of the ADF4351 data sheet: http://www.analog.com/static/imported-files/data_sheets/ADF4351.pdf#page=21Or maybe take a look at the ADF4351 evaluation board control software source code: http://ez.analog.com/message/52311

Here's the relevant section: