Agreed. "Torture" as a concept doesn't describe any particular experience, so you can't put a specific pain level to it. Waterboarding puts somebody in fear for their life and evokes very well-ingrained terror triggers in our brain, but doesn't really involve pain (to the best of my knowledge). Branding somebody with a glowing metal rod would cause a large amount of pain, but I don't know how much - it probably depends in the size, location, and so on anyhow - and something very like this on a small scale this can be done as a medical operation to sterilize a wound or similar. Tearing off somebody's finger- and toenails is said to be an effective torture, and I can believe it, but it can also happen fairly painlessly in the ordinary turn of events; I once lost a toenail and didn't even notice until something touched where it should have been (though I'd been exercising, which suppresses pain to a degree).

If you want to know how painful it is to, say, endure the rack, I can only say I hope nobody alive today knows. Same if you want to know the pain level where an average person loses the ability to effectively defy a questioner, or anything like that...