We use Total N, P2O5 and K2O values.  Most analyses will show these values in actual numbers or %, and then in values per ton or values per 1000 gallons.   Look for these - values per ton or values per 1000 gallon, whichever is appropriate - and go from there.

I figure only a portion of the Total N is available, but I figure 100% of the P2O5 and K2O to be available.