Hi there,
I took a look at the file. Remember the order of mathematical operations? I looked it up (thanks to http://mathforum.org):
"... you might remember this as "Please excuse my dear Aunt Sally.

1. Parentheses
2. Exponents
3. Multiplication and Division
4. Addition and Subtraction
"
Put the -1 in brackets and your problem is solved.

off = (-1)^IDX1
if off <1
memb_v = (lead_v)*(2)
else
memb_v = lead_v
endif

Greetings - Chris