Bearing in mind that I started when there was no telemetry, when you only fly with the timer of the transmitter. Flying close you learned the total battery time and then extrapolated it to the flight time. Later, we received the voltage thanks to the telemetry. There weren't that many percent, there was either total voltage or voltage per cell. In fact I still calculate it the same way and that's why I have asked many times that in the GUI we have the possibility to change between % of remaining battery and cell voltage. For cell voltage is easier.
I know that the fully charged battery is 4.2V per cell. I know that it is recommended to stop flying with 3.7V per cell (although I usually stretch to 3.5) and always remembering that under no circumstances can go below 3V per cell because it would kill the battery.
Maximum distance, coverage check or whatever you want to call it. I want to fly as far as possible, return and land with the minimum battery. As soon as I start to fly I take into account the altitude and the speed and direction of the wind. I know that this is what most penalizes the battery. I advance in one direction and if I have the wind against me, depending on the speed of the wind that slows the drone, I fly up to 60% and turn around. That margin, with experience, after having done many flights, you do mental calculations that are very close to reality, is increased a little if the wind increases (keep in mind that the wind, we can not predict many times in real time). When I land I know how much I have left and I know if I have fallen short and could have flown more or arrived too close. The next time I stretch the "PNR" or I shorten it.
In the same way you can do the same, without endangering the battery, making many trips back and forth but shorter. Calculate at what voltage the drone is going to land and see if with the current circumstances you have fallen short. It doesn't have to be specific with trips, it can be by recording trees, calculating distances between two objects, etc. You learn that if each drone is different, how much it consumes and each time you lengthen it more, little by little, until you get to land with the voltage you are looking for. I go from this tree to the wall starting with 3.95V and I have to go back and land with 3.82. I land with less than 3.82 and I know that next time I have to be more conservative, I land with more? I have fallen short. In the end it's very simple, it's based on repetition, training, experience, whatever you want to call it.
I don't know if I've explained myself well, I find it difficult to explain this and especially in English, but in the end it's a game within the flight. In the beginning you have to make the effort to remember to do it every flight and then leaves alone. Of course when you start it's better to stay short and reduce the margin of "unforeseen". Many times you arrive with more battery power than calculated, and you change it because it is not worth taking off again (this bothers me a lot, quite limited is the time of flight already). There comes a time when you calculate it quite accurately.
This is my method, somebody probably has a better one, I'm all ears