Hi Nick
Here's how you do it.
Create an Alternate Price List called 10%OFF. Then use the Create Alternate Price function and add whatever items you want to charge 10% off to this APL. When doing this, uncheck all but the Base Price box and select Percentage Decrease of 10% on the bottom row.
Whenever the base price changes or you add a new item, you can repeat the procedure, overriding the existing APL when asked to do so.
Special Prices is not the way to go here, Alternate Price Lists is.
Enter 10%OFF as the price list on the customer master for this customer.
Steve