The problem there is that the vast majority of C-14 is generated by the solar wind and cosmic rays converting Nitrogen (N-14) into C-14, which then decays back into N-14. There is no way to accurately measure the proportions using the decay products. That's why they measure the ratio of C-14 to C-12 and calculate on the assumption of relatively constant creation of C-14 (an assumption we already know to be invalid, but not, we think, hopelessly flawed). So, even though there are dramatic fluctuations in the rate of C-14 creation, such as the spike in the late 8th century where, IIRC, about double the normal amount of C-14 was present (calculated by analyzing the individual rings of trees that were preserved by submersion in an ice lake). So, not only is there significant variation in the levels of C-14 present historically, we don't even have a complete record of that (yet - it is a work in progress). C-14 dating is no better than a best guess based on reasonably solid, if incomplete, data.
There are fluctuations of no more than 5% in C14 production, but over the time period in question—C14 is good for dating back about 50,000 years—the fluctuations even out to a mean, which is actually a very good statistical basis on which to base calculations. Furthermore, the C14 creation in any year can be checked against other data, such as tree ring data, allowing for the C14 content of a sample to be calibrated to known variations.
C14 dating is not a “best guess.” On the contrary, because there is enough corroborating evidence from other sources, it is a very good method of determining the age of a recent (geologically speaking) specimen.