The DSP-610's frequency is controlled by a Silicon Labs SI-570 programmable oscillator which is really cool. You can read the calibration data from it to fine tune the frequency, it's complicated to do and I didn't do it so if anyone would like to I'll be happy to update the baseline code.
What I did is allow the base and step frequencies to be modified in the PIC18 code (main.c) so you can adjust the frequency in software.
Here is a spreadsheet that calculates the HEX values for the base and step values. The SI570 uses 38 bit math but fortunately the upper 6 bits stay the same (2) for all LO frequencies so it's hard coded in the I2C routine as are the HS_DIV and N1 values. The Step value is multiplied by 16 for better resolution, then after it's multiplied by the it's accumulated it's shifted 4 bits to the right.
The basic procedure would be set the radio to 49.99 MHz (RIT off) measure the actual LO frequency (in MHz) and then divide the result by 39.29. This will give you a very small number which will be very close to 1. Multiply the 114.20508997 in cell B! of the spreadsheet and this will give you a new Fxtal, put that in B1 and you'll have new numbers for the base and steps.
Remember that the Base is the 32 lower bits (8 hex digits) the upper bits will always be 2.
I hope this all makes sense.