Mike,
I am sorry I did not really pay enough attention when I wrote the reply yesterday.
the script in the parameter script window should go like this:
VALUES "lg" RANGE [0,h)
IF GLOB_MODPAR_NAME="h" THEN PARAMETERS lg=h-old_diff
IF GLOB_MODPAR_NAME="lg" THEN PARAMETERS h=lg+old_diff,old_diff=h-lg
the first line limits the lg values to 0<=lg<h otherwise when lg>h you would get the error message
::rk