The advantage of isochron dating as compared to simple radiometric dating techniques is that no assumptions about the initial amount of the daughter nuclide in the radioactive decay sequence are needed thus eliminating one creationist argument about radiometric dating.

Indeed the initial amount of the daughter product can be determined using isochron dating. This technique can be applied if the daughter element has at least one stable isotope other than the daughter isotope into which the parent nuclide decays.

Here's a link to a presentation of Isochron Dating methodology http://www.talkorigins.org/faqs/isochron-dating.html#generic .