You won't need to specify ferrite content on base metals.  304H base metal would normally be supplied in the solution annealed condition and contain little or no ferrite.  Ferrite is formed during cooling of the weld based on cooling rate and chemistry of the the weld deposit (i.e. a mixture of the filler metal and based metal).  You should order the filler metal with the specified ferrite content.  You would also want to estimate the percent dilution of the weld with base metal and check the estimated weld deposit chemistry against the WRC-1992 diagram to see if you would be expected to fall in the 6-10 FN range.  If your estimates show you could be out of range, you may want to raise or lower the specified FN for the filler metal to put it in the right range.  You could also do this prior to starting production by testing on weld samples of the 304H tube and the filler metal for the project and check the weld deposit with a Ferritescope or other ferrite measuring instrument.

What calls my attention is that gsi is required to check the FN on ALL welds.
If the base material, as Marty says and I agree, has little or no ferrite and the filler metal is specified to have a certain ferrite content, then the PREDICTED FN should be the same for all welds.
So, in my opinion, what gsi's client wants is to check the ACTUAL FN on all welds, after the weld is complete, by means of a Ferritoscope or other similar instrument.
Giovanni S. Crisi
Sao Paulo - Brazil