LT3800 datasheet
http://cds.linear.com/docs/en/datasheet/3800fc.pdf
See page 15, "Output Voltage Programming".
Output voltage is programmed through a resistor feedback network to VFB (Pin 6) on the LT3800. This pin is the inverting input of the error amplifier, which is internally referenced to 1.231V. The divider is ratioed to provide 1.231V at the VFB pin when the output is at its desired value. The output voltage is thus set following the relation:
R2 = R1 x (Vout/1,231-1)
Best regards,
Peter