Jason's library has a very good calibration procedure found by clicking on 'File\Examples\Etherkit-si535\si5351_calibration'
You will need some way of measuring frequency. Once done you will end up with a number that will replace the 0 in this statement:
Initialization now takes two arguments in the new library. Instead of just the crystal load capacitance - it now takes the crystal frequency as well.
A zero as the second parameter defaults to 25Mhz, as above.
If you're using a 27Mhz crystal you could change it in the header file (si5351.h) or just put in 27000000.
Any and all comments are appreciated.