then don't do it in DAT file, make changes to inline forms in SRC using HMI editor.
if your program already uses base 18, then keep it as base 18. this can be the working base.
but remove anything that alters base 4. this could be your reference base data (measured).
then at the begin of your program copy base 4 to base 18 so you always start with clean slate bit never modify reference.... you can manipulate base 18 all you like. next program start will bring it to initial value again.
also set $ADVANCE to 1 so you can see changes early.