The output voltage is determined by the resistive divider feedback network connected across the output, in conjunction with a 1.6 VDC internal reference on the LT3757 (for positive outputs). See the
datasheet.
Google is your friend here. The Linear Technology web site has extensive application notes, datasheets, and LTSpice simulations that will help you design a boost regulator. Most of these circuits are designed to operate with a fixed, regulated, output voltage and a widely varying input voltage. It is virtually impossible to design a boost regulator to operate with an output range of zero to some maximum specified output voltage. You need to specify a single output voltage and load current and proceed with your design from there. If you really need the output to be variable from zero to thirty volts, say for use as a laboratory bench power supply, there are other circuit topologies that are more appropriate.
