discount calculation... with a server session variable.

Hello there,
I've set a Session variables on my Ecart. This variable is related to a field in my "customer" table that defines the discount that customer is entitled to (the field is set to varchar, but I think better to change it...).
By the way, how can I set up the calculation on the ECart wizard in order to have the Subtotal price deducted of that discount variable?
All the procedures explained I found are too clumsy and not clear, so a sort of step by step would be the best.
Just for the sake of the example, the Session variable name is "CustDiscount".

Thanks in advance for your help

Michele

on the discounts tab, crate a new discount rule.

set the trigger to use:
Session Variable 'CustDiscount' exists

AND
total number of unique items in the cart > 0

set the calculation to use the Based on cart subtotal condition:
Subtotal of cart 'times' '$_SESSION['CustDiscount']'