I go by the recommendations here: How to prolong lithium-based batteries
Their recommendations are towards the bottom of that page.
A couple of things to note about Li-Ion.
The chemistry of the battery doesn't suffer from "memory effect" like Nickel-Whatever solutions so partial charges won't permanently or semi-permanently reduce the charge capacity of the battery.
But, Li-Ion batteries do need electronics to guard against over-charging and over-draining. These electronics calibrate themselves by measuring the decay in power as the battery drains. They need at least one full cycle of draining and recharging all the way to both extremes in order to get a fix on the capacity of the battery. Do one such full cycle early on to set up the calibration and then again every 30 or so partial discharges to maintain it.
The article even says in its recommendations that you should avoid frequent full discharges...