I start with 57 bundles of flax fibre, and I start to spin it with a spindle.
(143270):99h0:[#]{0347055F} | TEXTILECRAFT OPTIONS: Spin yarn
(575757):99h0:[_]{0347055F} | You can spin 100 m of yarn on one go.
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I select the maximum, which uses up 20 bundles of flax fibre and produces 14.7 metres, not the promised 100 metres.
I do this again and get another 14.7 metres of linen yarn.
With my last 17 bundles of flax fibre, it tells me I can produce 85 metres and I successfuly do this. So I get a lot more yarn out of the 17 bundles of flax than I get out of 20 bundles of flax.
My 14.7 metres of linen yarn weighs 0.6 kg and my 85m of yarn weighs 0.5 kg, so I think there is something wrong with the way the lengths are displayed.