My general point was that one can make a mathematical model that takes into account the historical value of the ambient C-14 to C-12 ratio, plus the historical

My general point was that one can make a mathematical model that takes into account the historical value of the ambient C-14 to C-12 ratio, plus the historical value of the C-14 decay rate. C-14 dating isn't used for dating recently-dead things, but not for the reason you suggest. The real reason is that the half-life of C-14 is about 5730 years. Not much decay happens in 200 years, and the little that does happen is difficult to sort out from measurement errors. Other techniques are used for dating things that died recently. Watch CSI for some clues :D