Charlie, the FDC dilution is essentially the same 1+4, Troop has used a weaker solution of Metaborate, also he's taken Crawley's figure of 1oz Metaborate in 12oz water (83.3gm/litre) and and substituted 50gms Metaborate/litre. Perhaps you took the 1+3 figure from the 1+1+3 Troop gives as the working solution.

Ian