With some chains, like Campy's, measuring only elongation is worthless, since the chain rollers and side clearance wear far faster. Using a 12 inch precision rule is plenty accurate, since 0.5% is a totally arbitrary amount of elongation that someone decided to advertise as the best time to change a chain. While changing at that point will make the cassette perform longer before new-chain skip occurs, there's no proof that 0.4% or 0.6% isn't better. If several chains are used in a rotation, each chain can be used longer, since new-chain skip will never occur and each chain could be used until 1% elongation was reached, IF it's the type of chain that wears primarily by elongation - not all do.
Well known Jobst Brandt postulated that only using a chain with too much elongation would cause new-chain skip, but I proved that wrong by using a Campy 10 chain for 6000 miles on a new cassette. The chain had less than 0.25% elongation at that point, but a new chain skipped on the two most worn sprockets, due to the wear pockets the worn rollers created. A chain with only a few hundred miles of break-in wear did not skip on those same worn sprockets.
https://yarchive.net/bike/sprocket_wear.html