Yes. The goal is to maintain the same resolution at 0-5V for the entire range 0-24V. A resistor divider isn't going to work. For 0-5V the entire ADC range can be used, but for 20-25V, only the top 20% of the ADC would be used.
It is a 10 bit ADC. I would like to maintain the 1.2mV resolution, at worst 6mV. I hoping to get a wider spec soon, but best to aim high to start. Don't you think?
I chose the 4.5V steps, so as to provide space for hysteresis in changing offsets and avoid getting to near the rails. 4.5V is not fixed, but easy to calculate. I was going to provide hysteresis thresholds 100mV apart and 100mV from the rail. In a voltage rising scenario, upon reaching 4.7V change up an offset level, that would then provide an ADW reading of 0.2V. If the ADC reading then drops below 0.1V, then drop down an offset level making the ADC reading 4.6V.
It just occurred to me, they make 18V and 9V voltage regulators. That would provide two of the steps, 4.5V is half of 9, a simple resistor divider, 13.5V is more tricky but easily derived from the 18V regulator.