The 50% recommendation is for storage for 6 months or more. It has nothing to do with batteries in regular use.
There is no "optimum" percentage. Too many people are making up nonsense as they go, misleading those who are looking for useful information. The best use is to forget about the percentages and just use the Mac, running on a natural mix of AC power and battery.
If this was true, why are the manufacturers of Lithium batteries saying 50-80%?
And why are many other brands then apple mention those exact numbers, and even have software that locks the charging to no more then "a number between those".
If there really where no substance to this claim at all, there wouldn't be these kind of settings, since people for sure rather have 100% of their battery when they need it.
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
"Commercial chargers do not allow changing the charge voltage limit. Adding this feature would have advantages, especially for laptops as a means to prolong battery life. When running on extended AC mode, the user could select the long life mode and the battery would charge to 4.00V/cell for a standby capacity of about 70 percent. Before traveling, the user would apply the full charge mode to bring the charge to 100%. Some laptop manufacturers may offer this feature but often only computer geeks discover them."
http://lancair.net/lists/lml/Message/56976-02-B/Li-Ion%20Battery%20Life.pdf
"Using partial-discharge cycles can
greatly increase cycle life, and charging to less than 100%
capacity can increase battery life even further"
"Unfortunately, charging and maintaining the battery near these levers (100%) shortens battery life. One solution is to select a lower float voltage, which prohibits the battery from achieving 100% charge.
...Reducing the float voltage by 100 mV will reduce capacity by approximately 15% but can double the cycle life."
"the battery is only charged to approximately 85% of capacity, which is an important factor in battery life."
(a charger) "can provide a longer battery life by selecting the correct charge-current level at which to en the charge cycle.
A C/10 termination level will only bring the battery
up to about 92% capacity, but there will be an increase in
cycle life. A C/5 termination level can double the cycle life
although the battery charge capacity drops even further to
approximately 85%"
"For maximum run-time, the charger must
charge the battery to 100% capacity. This places the battery
voltage near the manufacturers recommended float voltage,
which is typically 4.2 V ±1%. Unfortunately, charging and
maintaining the battery near these levels shortens battery
life."
"Avoid charging to 100% capacity. Selecting a lower float voltage can do this. Reducing the float voltage will increase cycle life and service life at the expense of reduced battery capacity. A 100-mV to 300-mV drop in float voltage
can increase cycle life from two to five times or more."
There are hundreds of these articles on the net. And many of them are really scientific.
And that's why many brands have software to lock charging to less then 100%, usually between 50-80%.
If there where no truth to this, there wouldn't be such software, and it most definitely would not be these studies about the gain you can have from it...