Are you looking for DPN or PHRF?
The DPN for a N6.0na is 62.6.
The DPN modifier for a spinnaker is 0.96, so your DPN is now 62.6*0.96 = 60.1
To converto DPN to approximate PHRF, PHRF = 6 * DPN - 330
6*60.1-330 = 30.6 which gets you very close to what the US Sailing table says, here
http://www.ussailing.org/portsmouth/table_v.asp