Exam Certified CPQ Specialist topic 1 question 148 discussion

The admin at Universal Containers set up a Price Rule to override List Price with a discounted promotional price. The Price Action has a formula which is as follows:
SBQQ_ListPrice_c* (1 – Promotional_Discount__c)
The admin is finding that every time Calculate is clicked, the price is adjusted. For example, if List Price is $10.00 and promotional discount is 10%.
10.00 * (1 – 0.10) = 9.00
The next time Calculate is clicked, the following calculation takes place:
9*(1 – 0.10) = 8.10
If List Price must be overridden, how can this problem be fixed?