For those still trying to figure this out, like me, this is the answer:
First change the custom price attribute as described above.
Go to the price rule, in the action menu, find the section "Apply the rule only to basket items that meet the following conditions (leave blank for all elements)"
Set conditions for
If ALL of these conditions are FALSE :
Special price equal to or greater than 0.01
therefore, the discount applies only to products that are NOT special. I found that every other way threw a mistake when there was a mixture of special prices and full price products in the basket.
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