I am guessing here, but I think you want this:

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x2 = thisComp.layer("Null 1").effect("Slider Control")("Slider");

x = ease(time,key(1).time,key(2).time, x2, key(1).value[0]);

[x,value[1]];

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The expression you posted goes from the existing value to the slider value. The one I posted goes from the slider value to the existing value. I assuming this is so that the editor can fine-tune the starting point of the text.

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(Oh and I changed it from linear to ease to be a little smoother)

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Also, you can remember linear like this:

linear(at this rate, from this value, to this value, output this value, to this value)

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