LED Strip - Voltage Drop when Wired in Parallel
The voltage drop in a segment the supply wiring depends on the current in that segment.

The current in the segment from the supply to the top LED strip will carry the full 8.4 Amp, so you would use that current to calculate the voltage drop in that segment.

The 0.3 m to the second strip will only carry 7.8 Amp, and so on down the system.

The voltage drops in the 0.3 m segments between the strips may be small enough to ignore (I don't use metric wire, so haven't done the calculations).

The voltage at the input to the last strip will be the sum of voltage drops in all the wire segments feeding it.

There will also be voltage drops along the LED strips - you may want to run wires to the mid-point and far end of the longer strips to reduce that voltage drop.