You did good! Precisely the info I was looking for. And thanks too for the tips on technique.

Along the way, you convinced me to try the wood/string version. Your explanation suggests that if I were to experience a tiny bit of piston shrinkage, a slight bulking-up of the string should compensate to maintain integrity of the seal.

Guess Darrel's about to get more of my money. I'm very pleased with the firesteel/flint kit I bought from him couple of months ago. Quite satisfying to strike a fire the old world way.