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@TheShipwright
You need measurement a few things. Backend between the DOs is 120mm? Measure from inside of the DO to the centre of the sprocket/cog. So centre of backend based on 120mm is 60 and then minus the measurement from DO to cog poss 15 so you end up with chainline of 45mm. Now measure your seat tube width probably 28mm and then measure from from edge of seat tube to the centre of crank. If the crank is on the correct chainline it will be 31mm from 31mm. Does that make sense? Your basically taking everything back to the centre of bike.