Hi,
Well, the basic answer is to take the tuning value and divide it by 2^32/125M or, in this case 17179869/(2^32/125000000) or, in short, 17179869/34.359738 = 500000.
Now, how to do this in PBP depends on if you're able to use LONG variables or not and to what accuracy you need the result. I don't know if you've seen my Wiki-article regarding the AD9850 or not but it describes two possible ways to calculate the tuning value when you know the frequency, it should be pretty straight forward to adapt any of the two methods to do the reverse.

/Henrik.