To use on 12 to 18 volt input for 12 volt out do the following:

Change R7 to 10K
Change R11 to 240K
Change R5 to 2.7K
Add R16 1.1K in parallel with C5
Add D4 22 volt zener 1N4748 in parallel with C4 (IE across power)
Add D5 22 volt zener 1N4748 from drain to source on Q1 (across the
MOSFET)