Modifier percent to discount amount not unit price

I have been able to repriice a contract sub line with an amount and have the sub lines and lines calculdate correctly. However I am trying to have a modifier kick which would then discount by a percent the  amount of the contract line. 

I have been able to develop sourceing rules so the right modifier qualifies but the modifier is taking the discount of the unit list price not the dollar amount of the line.

Is it possbile to have a modifer take 10% discount off a contract line dollar amount of 1000