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The AD9834 frequency output is as accurate as out 100 MHz reference clock.
If we set the DDS for an output of 12 MHz and the reference clock is 200 Hz high, then out 12 MHz output will actually be 12,000,024 Hz.
To compensate for this error, all we need to do is rewrite the FreqHex: table at the end of the source code at http://w8diz.com/qq-fc-project/ATmega88PA/ATmega88.asm.

I wrote a web program that calculates the new table values based on using the 100 MHz clock and measuring the DDS output freq when set for 10 MHz.
Go to http://w8diz.com/qq-fc-project/ATmega88PA/calc-9834.php to view the program.
Power up the DDS, which defaults to 10 MHz and measure the actual frequency output.
Enter the measured frequency (in Hz) into the field labeled "Enter measured frequency (Hz):" and click on "submit".
Cut and paste (replace) the new FreqHex: table into the source code and recompile the code.
Upload the resulting HEX file to the DDS target device and the result is a calibrated DDS output.

The only error from now on is a result of temperature changes to the 100 MHz clock which will result in some drift.