FMS for inter-linked Percentage and Amount fields

Hi,

In A/R Invoice Freight window, I want to add a UDF for Freight %. If user specifies a %, the Freight Amount should get auto calculated. If the user changes this calculated Freight amount, then the Freight % should get recalculated.

This is similar to the Discount functionality (document level discount) in SAP.

How can I achieve this using FMS or other means?

Thanks.