How do we know radiometric dating is accurate
Other methods do not require knowing the initial quantities.
For example, potassium decays into two different isotopes of argon having different half-lives.
Methods are precise insofar as they are properly used.
A good explanation of all of the dating methods used for samples up to about 200,000 years old, together with their accuracies and references to the scientific literature is contained in  Walker, Mike, "Quaternary Dating Methods," John Wiley & Sons, 2005
Every few years, new geologic time scales are published, providing the latest dates for major time lines.
Older dates may change by a few million years up and down, but younger dates are stable.
By analogy, a stop watch will not keep accurate time if it is not wound, if it is not in good repair, or if the operator forgets to press the button.
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.