Surely this assumes that the test (collection and analysis of data) is working properly, and the major point that seems to have come out of Froome's case is that it isn't?
If the current test is inadequate you don't look to change the rules to account for this, you change the test.
The test is perfectly adequate, in the sense that it accurately measures the salbutamol level in the urine. The problem is that the relationship between that level and the amount the athlete has taken can vary widely, depending on how much he inhaled, and when. This is just the messy nature of pharmacology, and isn't the fault of inaccurate testing.
Though I was never able to get the details, the gist of Froome's argument was that a) the urinary levels following a specific dose are logarithmically distributed, rather than randomly (normal curve) distributed, resulting in much more frequent outliers at the high end than WADA predicted based on its studies; b) Froome was inhaling every day, so the cumulative effect of the doses would be greater than assuming a single dose; c) the inhaler Froome used results in higher levels per dose than the inhalers used in the WADA studies; and d) the conditions of racing, and particularly dehydration, may have altered the dose-level relationship as generally studied in the lab.
The first of these, the logarithmic distribution, guarantees difficulty in setting a criterion that strikes a reasonable balance between false positives and false negatives. The immediate implication of this is that the cutoff urinary level should be set higher, but if you do that, there will have to be more false negatives. The only way to avoid this is by addressing each case individually, which brings in b) and d), which can only be considered if the athlete has detailed records. Froome's scientific expert, Darren Austin, made use of how much Froome inhaled every day (which almost certainly was guesswork, since Froome would not have kept the records), and factored in dehydration. By dehydration, I don't only mean the effect on the concentrations of substances in the urine, which is factored in already now, but the effect on metabolism in general.
My own conclusion, based on the data I was able to get, is that the probability of Froome's level was still quite low. I think in the end WADA accepted this because they felt that he would not have risked a positive by taking salbutamol orally, IOW, while the odds of getting this level following inhaling within the allowed dose limit was quite low, they felt the alternative possibility was even less likely.