First of all confirm the stretch by measuring 12" of lightly tensioned chain. The tools tend to read high, so a 12" stretch measurement will be a bit more accurate. On a new chain with 1/2" pitch each pin will line up exactly half an inch apart, with stretch 24 links, which should measure 12" will be slightly longer. Replace the chain when the stretch is more than 1/16" over 12", or 1/2%.
I never replace the cassette when replacing the chain, unless the chain skips. This is likely, but not assured, if the stretch is more than 1/8" over 12". So depending on how stretched your chain is, you might want to go ahead and buy the cassette in case you need it, but hold off changing it unless it's necessary.
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