Having a debate with my finance team, we staff to forecasted calls yet our finance team insist on building cost based on calls handled. Is it normal practice to build a finance plan on a straight 6.5% removed from call offered to determine billable m

Having a debate with my finance team, we staff to forecasted calls yet our finance team insist on building cost based on calls handled. Is it normal practice to build a finance plan on a straight 6.5% removed from call offered to determine billable minutes?