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Why use a formula/driver value rule?
The Formula/Drivers value rule enables you to create custom mathematical equations to calculate the values in your forecast’s Profit & Loss.
You can use any chart of account line items, subtotal, heading, drivers, or typed values combined with the 4 operators and parentheses to build formulas for any chart of account line items on the Profit & Loss.
Through formulas, you can acknowledge the relationship between P&L accounts and incorporate nonfinancial data into your forecast through drivers.
Drivers can be set up from the Fathom main grid and are essentially ‘nonfinancial’ metrics or leading indicators that calculate account values.
Examples: A driver for the 'Number of patients' can be used in a forecast for a dental or medical practice. A construction company could use a 'Number of contractors' driver. A SAAS company could use cancellations or churn rates as drivers. 
Creating a Formula/Drivers Value Rule
To create a new 'Formula/Drivers' value rule or edit an existing one:

💡Pro Tip: Formulas can be edited or overridden at any point, by opening the properties panel to the right of an account and using the ‘Three dot’ icon in the value rule section to bring up the editing options. You can edit the rule to change the formula or start a new rule to override it.
Examples
A couple of the many ways you can use the ‘Formula/Driver’ value rule to determine your forecast’s values include:
Linking the performance of one account to another
With a 'Formula/Drivers' value rule, you can base the forecast performance of one account, heading, or classification off of another account, heading, or classification.
Example 1: A company knows its ‘Materials’ expense account trends at 35% of its Sales. This can be represented in the forecast by building out the following formula for the 'Materials' account:
‘Sales’ x .35 
💡Pro Tip: You can turn on account codes to help you differentiate between accounts and headings when building a formula value rule.
Using a driver as a variable in your formula
You could set up and import data for drivers and use these drivers as variables in a formula. Some common forecast drivers include ‘Number of Units Sold’ and ‘Average Sales Price’. Using a formula you would be able to reference these drivers to forecast sales according to the following formula:
‘Number of Units Sold’ x ‘Average Sales Price’
Example 2: In the example of a coffee shop, the data for the number of 'Hot Coffee Purchases’ and ‘Iced Coffee Purchases’ could be entered into the Drivers Grid. Additionally, drivers for the ‘Average Price per Hot Coffee’ and ‘Average Price per Iced Coffee’ could be created. Then, using a ‘Formula/Driver’ value rule, the Sales account could be calculated according to the following formula:
(‘Hot Coffee Purchases’ x ‘Average Price of Hot Coffee’) + (‘Iced Coffee Purchases’ x ‘Average Price of Iced Coffee’) 
Additional knowledge & common questions: