You can define Billable Units on a price option. 

Under Management - Billing - Pricing

Select the plus sign to add a new price option OR select an existing option.

You will see a Billable Units field. 

Under the value you can indicate the applicable multiplier for this price option. 

The Billable Units defined here will be used to calculate the CHARGE amount and the QUANTITY when creating an invoice. 

Charge = (Items delivered * UOM) / Billable Units * Price Option

Quantity = (items delivered * UOM) / Billable Units 

Units Of Measure (UOM) are defined on the product catalog level 

Example: You sell a single product that has a defined 300 Calorie UOM.

You need to bill $x for every 100 calories provided when delivering the product. The invoice needs to calculate a charge per 100 billable units to display total charge and quantity.

In the billable units under the price option you would put a value of 100

In the UOM field in the product catalog you would put a value of 300