Seperate FTE data for Benefits premium calculations

We have a group of employees that are paid as full time employees and therefore have an FTE value of 1.0.  However, they are contract employees and may only be contracted for 6 months.  Under our benefits plans they are eligible to participate in a healthcare plan, but at a prorated premium, based on their contract length.  Is anyone aware of an existing data field that could capture this information which could be used in the formulas for calculating the employee premiums?  We currently use the primary assignment FTE for these calculations as part time employees are subject to similar