For anyone interested in this sort of thing there is another drug test calculator/widget available at Drug Tests Direct which includes other drugs and sample types, in addition to THC.

Thing is, since its the metabolises of THC that are measured when undertaking a screening test for marijuana the true detection times are dependant on your own metabolism, which can differ significantly between people.

Best way is stay off the grass well ahead of time whenever possible and do a home test prior to the screen to put your mind at ease.