First you lookup the resistance per meter of the wire you are using. There are many on-line resources for this. If it is a pure copper, solid wire then the resistance is 0.0075 ohms per meter.
For each loop, multiply your total wire length in meters by the 0.0075 ohms per meter. This will give you the resistance of the wire for that total length. Make certain you include the wire length from both sides of the LED strip.
Since the wire resistance will be much smaller than the load resistance, it is a matter of applying Ohm's law for a series circuit to get a good approximation of voltage drop across the wire. You know the total current in each loop (6 amps according to your drawing). You know the resistance of the wire in that loop from the earlier calculation. Voltage = I*R so multiply the current times the wire resistance and you know how much voltage you are dropping in each of the wire segments. For example, in your 18 meter loop (assuming it is 36 meters in total), the voltage drop in the wire will be about 6 amps * 0.0075 ohms/meter * 36 meters = 1.6 volts.
You should note that you have LED strips in series connected to the supply and others are by themselves connected to the supply. Unless these two strips are rated differently, you cannot do that. You will either burn out the single strips or the series strips will be too dim. Can you post a link to the datasheet for the LED strips for additional assistance with this?