From the site
http://www.efunda.com/units/convert_units.cfm?mode=short&From=387
the conversion is 1 foot-pound to 1.355818 Newton-meters. Therefore, 21 N-m is 15.488 foot-pounds and the valve cover bolts at 8.5 N-m are torqued to 6.27 foot-pounds.
Do you go to 15 foot-pounds or 16 on the rockers? I would probably go to 6 on the cover bolts.