Measuring the ratio of C14 to C12 and C13 therefore dates the organic matter for periods back to about eight half-lives of the isotope, 45,000 years.
After a long enough time the minority isotope is in an amount too small to be measured.
For example, the C14 concentration in the atmosphere depends upon cosmic ray intensity.
To take this into account, a calibration curve is developed using other dating methods to establish the C14 levels over time.
The actual accuracy of radiometric dating is about 2%, but there is no point in splitting hairs for this debate as to whether it is 2% or 3%.