I am an avid reader of eBooks and I do that mostly on an iPad. But there are many people who warn of deterioration of eyesight due to continuous exposure to the screen of an iPad. But I can't give up this habit. So what are the best settings I can use on an iPad to ensure my eye sight is saved in the long run, somehow.
Any backlit screen isn't optimal for reading; an e-ink screen such as that on the Amazon Kindle is preferred. But to answer your question...
The 20-20-20 rule: Every 20 minutes, take your eyes off your computer and look at something 20 feet (6 metre) away for at least 20 seconds.
I like to use the 204 rule, which is the above, but ALSO using the recommended 20 inches (50 cm) between the monitor/tablet and your eyes.