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I'm 75kg and I can make a Mercian in standard dia. tubing flex to the point of feeling too soft. My Argos Racing frame in the same material (725) has a 31.8 DT and has that steel 'buzz' without feeling too soft. All depends on rider weight, tube diameters, size of the front triangle (e.g. compact or not) and (to a point) butt length. Steel frame 'flex' has next to nothing to do with chainstays (why do people think this? I couldn't design a stiffer structure than a bike's rear triangle if I tried - it's a small, steel 4 sided pyramid) and more to do with the DT and TT being able to twist to allow the BB shell to kick to the side.
I'm just typing this nonsense because it costs a lot of money in frames to figure this out.
Always liked a bit of panto at Christmas.