thanks bill for distilling the more understandable bits of the wiki entry.i note that significant variation of carbon 14 is acknowledged but that recalibration is effected by recourse to other attempted methods,as tree rings and varves.this of course depends on the interpretation of that data.i was not able to locate any info in that article that disclosed that method.
the assumption is that there is little,if any ,variation in those methods over the millenia,and that the estimate of the rate of varve deposits,tree rings annually,etc are relably accurate.
as you might suspect- CMI scientists have tackled this before and published their findings on their site.