Electrical charging to 60%-80%

I have read in more than one place that charging chargeable devices only up to 60%-80% of their capacity makes their batteries last longer. I wanted to know how much longer.  Would such a practice matter?  I found one estimate that limited charging would double the life of the battery.  That is enough for me.

I thought I would find it easy to do and I did find that some devices can be set to a limited charge.  One electronics salesman said that a limit would have to be set by an app or section of the device itself and advised me not to bother asking about the availability of an external charger that could be set to go only to a limited portion of the charge.  

Maybe making the battery function longer would not be good for some businesses.  I have been trying to limit the charges on my devices.