
one half link on a typical derailleur chain is ~1% extension and both chains ae well past that. 0.75% is about the most that you can get away with if you want to re-use the freewheel. To be really safe call it 0.5%.
Suggestion; once the new chain is worn to about 0.4%, refit the old freewheel and run it for a week or two. Then try it with a new chain. If you are lucky it shouldn't skip with the 0.4% chain, and having been worn to match that, it ought to take a new chain OK too.
cheers