Just piping in to say I'd also like to see - in your own words - your problems with that method.
Assumption of initial conditions (all “parent” elements present?)
Assumption of constant rate of decay
Assumption of non-contamination (gain of material)
Assumption of no leaching (loss of material)
Varying results within one sample in current tests
Remember, no one was around to verify the assumptions above, and there’s no way to verify those assumptions.