I have no way to test if this helps you, but I found this in ifi_aliases.h It leads me to believe that you should scale the readings you get back by the same amount as IFI does in this code.
Code:
/*
*-----------------------------------------------------------------------------------------------------
*---------- Aliases for Battery Voltage bytes --------------------------------------------------------
*-----------------------------------------------------------------------------------------------------
* Formula to calculate voltage in volts:
* current_voltage = battery_voltage * 0.038 + 0.05;
*/
#define battery_voltage rxdata.rc_main_batt*15.64/256
#define backup_voltage rxdata.rc_backup_batt*15.64/256