The reason for stopping at lead is because lead is not radioactive and will not change into a different element.
Thus all the lead in the zircon can be assumed to be radiogenic.
The three main parameters that have to be set are the original amount of uranium and lead in the sample, the rate at which uranium and lead enter and leave the sample, and how much the rate of decay changes.
Uranium-lead dating uses four different isotopes to find the age of the rock.
Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.
After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.