Consider 20% is the same as 0% and recharge it.
Contrary to what some might say, LIPo batteries have an "ideal" break in period. First ten cycles or so, don't discharge down past 40% of the battery's capacity.
LiPo batteries do NOT like to be dropped down to 15% or so often.
Never let your Macbook die from power, you can corrupt files that way, and the batteries do not like it.