Answered by DrAzzy -
Per the schematic, the Vin input on that board (the barrel jack one) goes through a schottky diode to the "5v" rail, schottky diodes have a drop of ~0.3v. ...
Currently the only wires I have hooked to the board are +V (from +5VDC on the MW), GND (from the -VDC on the MW), and the 5V on the board running to a blank terminal block (nothing connected to the other side.)
Originally I had the A0 input hooked up across a potentiometer to simulate the sensor and when I set the supply to 5.009v as measured by a multimeter (fluke 117) the A0 input would reach 1023 at 4.66 VDC and I would lose the range from 4.66-5v.
The potentiometer was hooked as follows: supply to +DC from psu; GND to -DC from psu; and reference running back to A0 on 4duino.
I then tried switching the input to the potentiometer from the psu to the 5v pin on the 4duino and noticed that the max the potentiometer would read at this point was 4.66V (the psu still measured 5.009V with no noticeable fluctuation)
I then turned the voltage on the psu down to 4.663V and noticed the A0 pin would only read up to ~4.35V and the 5v pin measured ~4.35V as well. After noticing that the 5V pin followed the max that the analog pin was reading I disconnected the everything except the power to the board and the 5V pin wire and with some trial and error found that if I set the voltage on the supply to ~5.363 then the 5V pin will actually read 5.000V and my A0 will actually be able to read 0-5VDC.
Am I missing something here or is this intended usage and I will just need to supply 5.36V to the board to read the full range?