If an old chain had elongated by 1/2", then the ends would not match up with a new chain. You have to be smart enough, not to cut a chain so the ends won't mate together. If using an old chain as a guide, lay it side by side with the new one. It's easy to see if the old one is so much longer that it's far out of sync with the new one. If changed at .5% elongation, then the old chain would only be about 1/4" longer than a new one.

Just measuring the overall length with a tape measure will always work, if you record how many inches of chain are needed for a bike.

Use the little/little method to get the longest possible chain and get the maximum RD wrap capacity. The big/big plus 1 inch method certainly works, but suggests the shortest possible chain. That's OK if you never plan to put a different cassette on the bike with a larger cog.

Counting links is a complete waste of time.