I tried your values and it's doing exactly what I said it would do. You are missing the - in front of the first number.
Sinistralis -- Am I understanding you correctly?
The Damage Range value should be a negative value?
Damage Range
-50
If I do this, the output damage of the skill will range from,
800-1000 (random numbers) vs 997-1003 with the default Damage Range of 5 or 10 on skills?