Just a quick post for anyone who may find it helpful.
From what I have found by lots of searching it appears that the hexadecimal values in board-sholes.c represent voltage by the following formula:
(value as decimal) * 12.5 + 600 mV.
This seems to make sense.
Stock speed of 550 mhz uses 0x38 as the voltage value.
56 * 12.5 + 600 mV = 1300 mV
This *could* be wrong.. I'm not entirely certain and if anyone knows differently please tell me.
I found this info here