If you omit the 3.6k in series with the 10k potentiometer you can run from about 32 V instead of 42 V.
You don't seem to have understood how R1 and R2, and R5 and R6 are used.
R1 and R2 form a voltage divider which takes a fraction of the input voltage. When that gets to 1.25 V, the LM5002 turns on. If you want to run it off 12 V you should change the ratio of R1 and R2. You seemed to have changed their value and left the ratio the same. R1 and R2 have nothing to do with the current.
Similarly R5 and R6 form a voltage divider on the output voltage. The divider has to be arranged so that when the output gets up to 42V (or 32V), the divider outputs 1.25V. That makes the LM5002 turn off, therefore limiting the voltage.
In all voltage dividers, the ratio of the resistances is the most important feature. The value of the resistances is not important and can change over a wide range as long as the ratio is correct.