If you have a policy that earns premium/commission monthly this is the simplest way to set it up.  Set up the expected annual agency commission based on total premium, but then take monthly payments against it. For example:

Policy starts on February 15, 201. Annual Premium is $12,000. Annual (expected) Agency Commission is 10% or $1,200. Premium (and commission ) is earned monthly.

So, you receive a payment of $100 each month. your commission payments will look like this:

Agency Commission (Annual) expected based on premium $1,200:

payment received March 5 for February $100. Commission received $100, Balance $1,100

payment received April 4 for March $100. Commission received to-date $200, Balance $1,000

payment received May 6 5 for March $100. Commission received to-date $300, Balance $900

and so on