Populate Custom Column on a Revenue Arrangement with the discount from a subscription.

We have Suitebilling and ARM, I have the need to use the discount from the Subscription line in a fair value formula for allocation purposes. I have created a Custom Transaction Line field and it is displaying on the revenue arrangement, however I cannot get it to populate with the discount from the subscription line. When I manually populate the discount my fair value formula works , but I have to manually populate the value when it should be an easy task to map the value.

I have tried to create the field and source it from the subscription line discount and it will not populate, I know the screenshot below does not have the checkbox for store value, but I have tried it both ways, I have also removed the sourcing from the field and tried to map it via the Revenue Field Mapping but I cannot get it to populate.