The attached source file is a complete single file program.  It has this line ...

fxdLogX = -6 * (X - 5000) * (X - 5000) + 120040 * (X - 5000) - 1682360000;

It appears two times in slightly different contexts.  The code generated for each appearance of this expression is quite different.  The results computed are unexpectedly different.  

Build the program ...

% cl6x -mv6500 -g -s -o1 main.c -z -o main.out -m main.map 
<Linking>

When it is executed ...

Fail!
x = 2000
func_fail(x) = 439698000
func_pass(x) = -419296000
Assertion failed, (0), file main.c, line 66

The value 2000 is used because it does not cause the expression to overflow.  The func_fail and func_pass lines should show the same value.  The Assertion line should not appear.