Carbon dating always requires an estimate of the initial amount of C14 in the object whose age is being determined. That is an issue separate from the one being considered in this thread, but one whose error bar is probably more important. I cursorily touched upon that with:

  *x* is the initial amount of C14 we obtained via other means.

These 'other means' are thoroughly explained in various places on the web, due to the amount of creationist attacks on carbon dating.