Radio isotopic dating: Error margin 1% of what?

Radiometric or radio isotope dating is described as being accurate to a 1% margin. Is this always 1% of the half-life say 50 billion years, or 1% of the actual specific age revealed by testing that element eg strontium has an error margin of 50 billion years. So if I date an item at 50 million years old does that mean it could be 500 million out (1% of strontium’s half life) [!!??] or it could be out by 500,000 years (1% of that suggested date) – without consulting other means of testing such as index fossils etc.