To set up a standard rule for revenue allocation, see this article: How to set rule based revenue allocations.  

If you need to set up a two-step rule for revenue allocation, with some allocated in a fixed way by month and another element time-based, then select "Multi-Tiered Revenue" using the cog symbol at the top right of the Customers page. 

Then select the % of each invoice line item to be allocated to each rule.

• Set "Fixed %" to set a fixed % by month, starting from the invoice date.
• The remaining allocation will be date-based, spread evenly from start date to end date.