Calculate OCR2A from audio frequency
The equation to calculate it is given in the first article you linked:



The 2MHz value comes from the factor of 8 prescale on the 16MHz clock and 256 is the size of the lookup table specified by your LENGTH define. If you start from a frequency you want to synthesize, just use the above equation to solve for the necessary value of OCR2A. Note that since you have to input integer values to OCR2A, you can't really achieve arbitrary frequencies, but you're certainly not locked in to the values given in the supplemental musical note PDF you reference.

So, to solve for OCR2A:  

OCR2A = TCNT2_rate / (desiredFreq * wavetableLength);