1. First, make sure you are modeling your sales using “customer acquisition.” To do this, select assumptions/revenues/sales models in the menu. If the top of your screen does not read “Sales Model » Customers by Marketing and Sales Effort,” press the blue “Select a New Sales Model” button at the bottom of the screen and then select “BU 3: Use Marketing and Sales to Drive New Customer Acquisition.”
2. In the viral growth engine, emphasis is on customers referring other customers. Therefore, you should set the assumptions on this page as follows. In general, to focus on a viral model of growth:

• Expense Item or Employee Type: Any sales or marketing expense item or any employee type that drives sales (but make sure you have some dollars allocated to these to get the viral engine started).
  
  
• Customer Acquisition Cost (CAC): Usually can afford to be relatively high because you expect each acquired customer to spawn many other customers.
  
  
• Sales Cycle: Obviously the shorter the better, but like CAC, you might be able to survive with a longer cycle.
  
  
• Average Order Size (AOS): Obviously the higher the better, but your business is likely going to flourish on massive numbers of small transactions. So this will probably be a small number.
  
  
• Periodicity: Most likely monthly, but could be anything.
  
  
• Retention Rate: This could be a secret of your success. Higher the better (but some viral businesses (like open heart surgery), have zero retention rate but high viral coefficiency.
  
  
• Viral Coefficient: This is critical for your business. Must be greater than 1 for viral growth to work without constant influx of new cash.
  
  
• Viral Cycle Length: This is important for your business. Values less than 30 days create incredibly large growth numbers, and are extremely unlikely.

Related Questions:

What is a sustainable growth engine?

How do I change the customer retention rate? How do I change the customer attrition rate?

How do I change customer acquisition cost (CAC)?

How do I change the length of the sales cycle?

How do I change the periodicity? How do I change how often customers purchase?

How do I change average order size (AOS)?

How do I change the viral coefficient? How do I change the customer referral rate?

How do I change the viral cycle days?

How do I change the product mix? How do I change the products that customers purchase?

How do I choose prices?

How can I model the sticky growth engine?

How can I model the paid growth engine?