Rx Bandwidth?
You don't need to know fdev on the receiver side - just match baud rate and Rx bandwidth settings from Jeelib. There is a block in rf69.cp that has the parameter set up used at module init time.
static ROM_UINT8 configRegs_compat [] ROM_DATA = {
0x01, 0x04, // OpMode = standby
0x02, 0x00, // DataModul = packet mode, fsk
0x03, 0x02, // BitRateMsb, data rate = 49,261 khz
0x04, 0x8A, // BitRateLsb, divider = 32 MHz / 650
0x05, 0x05, // FdevMsb = 90 KHz
0x06, 0xC3, // FdevLsb = 90 KHz .......