Lookup is the standard functionality which provides the ability to access data that is within the system and then fetch the data to the appropriate budget Assumptions Sub-tab.  Non-system data can also be imported in the Assumptions Sub-tab using the Import Budget Assumptions screen.  Then using the Row-to-Row Calculations functionality, math functions can be applied to the accessed data. 

Please see the Lookup and Row-to-Row Calculation articles for more details

Please also review these videos for more information:

1) https://dynbud.freshdesk.com/solution/articles/13000004155-lookup-functions-financial-variables-

2) https://dynbud.freshdesk.com/solution/articles/13000004206-account-details-row-over-row-math-operators