With only 8' of wire the voltage drop will be small unless there is a bad (high resistance) connection in a block. Unfortunately the the testing done so far does not provide meaningful operational data. The voltages measured are "open Circuit" with only the test meter current draw which is at most milliamps. To know the track voltage during operation there needs to be a load on the block. For a 100W supply that would be up to 5A. However the power supply will only deliver the amperage needed to run the train, I suspect that is less than 5A. The power supply wattage needed is determined by the current draw of the trains you plan to operate.
Each of the power districts on my layout have a 10A supply, a bit of overkill. Two steam engines and a set of 8 passenger train can be in one block of a power district. The current draw for these two trains is between 6A and 7A (S gauge). With multiple 14ga feeders of about 20' I see about .3V drop.
