In fireforming a case with a light load in a chamber with unknown headspace, the case should be lubricated, allowing the pressure generated by the ignition to force the case head firmly back against the bolt face. You won't get a reliable measurement otherwise, since the firing pin fall will force the entire cartridge all the way forward and the adherence of the dry case to the chamber walls coupled with the low pressure will keep it from being forced back, resulting in a protruding primer and an inaccurate headspace reading.