Servive Management - Service Contract Duration

Explanation of concept:
In the service contract the "amount per period" is calculated as the "Annual Amount" / "Invoice period".
I want the contract duration to be six months. Thus the totals of the contract lines will be the total of "Line amount" of the 6 months, this will be the annual amount will be this total. Contract must be invoiced monthly thus the calculation will be "Annual Amount" / 12. So the amount used when Generating invoices for this contract will be exactly half of what it should be. From my investigation of the manual and the code the only way to get the results for only six months would be to to calculate the amounts for a period of 12 months. because the "expiration date" is not used to calculate the "amount per period"
Question:
Is there a way to setup a contract with a Duration less than 12 months, so values entered are accurate and the calculations of amount per period is accurate as well?