where did the 1024 come from in the 0.28/1.1 * 1024 equations?

It's the total number of steps in the Arduino's ADC, it reads 0 to 1023

so on a 3.3v Due the ADC would give values of:

Voltage ADC value

0v 0

3.3v 1023

1.65v 511

2v 620 (that's 2 / 3.3 * 1024 )

about the pull up resistor part

Yes, we've looked at two cases...

1) the input voltage is higher than Vref (originally 5v and 1.1v) where you want a potential divider to make best use of the ADC range so, as you say "you choose a resistor value to get reasonably close to the Vref at max resistance".

2) the input voltage is the same as Vref (now we have 3.3v)

Now we have to use some 'pullup' resistor to supply our sensor resistor so we can't use the full 3.3v range

If the pullup is very large (e.g. 10,000ohm) to our 200 - 800ohm sensor, then the voltage will be low with a low range. (0.06 to 0.24v)

If the pullup is really low (e.g. 10ohm) then we may draw too much current and the voltage will be high with a low range (3.14 to 3.26v)

Somewhere in the middle we get a reasonable balance, having the pullup be equal to the sensor resistance at 1/2 it's range is a good bet.

So... 470ohm pullup gives us 0.98v to 2.07v

Yours,

TonyWilk