@bikerevivesheffield 0.5% on 12″ is tiny, 1.5mm. try measuring across 5 or 600mm. Also tricky with a steel rule as there’s no anchor point. You’re trying to line it up by eye with the pin centre, or end of the link or similar.
The 0.8% thing is interesting and was mentioned recently by Adam at Zero Friction Cycling. It might be that due to the hard chrome finish on the chain, SRAM are suggesting you run chain and cassette to end of life together. Perhaps at 0.5% on the chain, your cassette is now too far gone to take a new chain. Would be interesting to find out.
However, when the chain gets to 0.8%, you will absolutely need a new cassette