to know where is the 50%, you kneed to know the minimum voltage , the maximum voltage and the discharging curve (that is certainly not linear)
so with min and max, you get the range (and 50% should be around min+range/2), but since the non linearity of the curve (battery discharge faster at the end) you can expect 50% will be more like 60%)
counting watts or Amps could be a solution, but unfortunately, you will not get same power from a battery if you discharge slow or fast, so again, the 50% could be a different value than the calculated one.
The easiest is to make a real life test, drain the battery from top to bottom once, then you get the time it takes with real use.
then you know that that if you get 6 hours of capacity, 50% will be 3 hours.
then you charge battery again, run then for 3 hours in same condition and measure voltage in the battery.