Currently I am using the formula by Tam et al 2011., 

TBW = [((T x A/a) x ((Ea – Et)/(Es-Ep)))/1000]/1.04,

where A is the amount of dose solution drunk (g); a, amount of dose solution diluted in T (g); T, amount of water in which ‘‘a’’ was diluted in (g); Ea, enrichment of diluted dose; Et, enrichment of water used to dilute the dose; Ep, enrichment of baseline sample; Es, enrichment of postdose sample; 1.04, correction factor for overestimation of TBW by the use of D2O. 

As the doubly labelled water solution that I've used came in a pre-mixed drinkable solution, I'm unsure about the values to substitute for 'T' as well as 'a'.