Experimental aircraft pilot operating in ZMA airspace reported a NMAC with another VFR aircraft that was traveling in the opposite direction at the same altitude. Reporter stated the area has an LOA that allows IFR traffic to fly at odd altitudes while northbound; and to fly at even altitudes while soundbound; which may be in conflict with the standard of eastbound headings being flown at odd altitudes and westbound at even altitudes.

Narrative

I was enroute southeast bound on T208 VFR; in VMC conditions; not with flight following; at 7;500 ft. Roughly around FORNI intersection located on V3; I was navigating on T208; roughly 3 or 4 miles northwest of MALET intersection; I received an ADS-B In traffic advisory for traffic 12 o' clock; less than 1 NM; same altitude. It is important to note that the traffic was not on my display prior to this alert. I saw the traffic on ADS-B appear on my map as if their ADS-B Out just started to work. Upon receiving this alert; I checked my map to determine the direction of the traffic; and recognized that it was in the opposite direction on the same airway at my exact altitude. I disconnected my autopilot and checked ahead to find the traffic to ensure that I don't take an evasive maneuver into the traffic in case it was slightly off course from where I was. I identified the traffic after a few seconds as Aircraft Y. I estimate that I observed the traffic closing at less than ½ mile at the time I found it. I recall that while it was a good VMC day - the sun was blocked to make the aircraft not very reflective to identify easily.Once I recognized that I was on a direct collision course I made an immediate right bank turn to avoid the traffic. I recall from the point I made the bank to the point it passed me was roughly 3 seconds. I continued to observe the traffic to determine if the pilot of that aircraft also took evasive action. They did not. I do not believe the pilot of that aircraft was paying attention; and is still unaware that anything happened on that flight. After recovering the aircraft and reestablishing a flight path with autopilot engaged; I attempted to contact the aircraft on 121.5 as I had its tail number listed on my ADS-B as Aircraft Y. Attempting to contact twice; they did not respond. I then contacted Daytona Approach on 125.35 who then directed me to Orlando Approach for the location I was in - I do not recall the frequency. Orlando Approach informed me that he was not talking to that aircraft. After the flight; I reviewed the ADS-B data online to estimate how close we actually got; as well as altitudes and ground tracks. I was able to verify we were on the same airway going in opposite directions at the same GPS altitude; and at my estimated point of closest approach; we were 300 to 400 ft. after my evasive maneuver. I firmly believe that we were on a nose-to-nose collision course.It is my understanding that there is an LOA (Letter of Agreement) for the eastern sector air traffic controllers in Florida that for IFR traffic heading northbound; they fly at odd thousand altitudes and southbound flies even thousand altitudes; which is in contrast to the normal eastern tracks being odd and western tracks being even. This is to avoid all airline traffic and IFR traffic from having to change altitude as they go north along the east coast.Due to this LOA; I wanted to clarify with ATC if there was a written document; NOTAM; or AC of some kind that applies this to VFR traffic as well. The ATC controller informed me that they do the reverse of even and odd thousand altitudes for IFR traffic; and they also do it for VFR flight following traffic. They said that northbound will be odd thousand plus 500 and southbound would be even thousand plus 500. This directly contradicts AIM 3-1-5.The controller further reminded me that VFR traffic can technically fly whatever altitude they want as they are not being controlled. I asked them for a reference specific to this swapping of altitudes; they responded with that it was only in the LOA amongst controlling agencies. I can recognize and understand the IFR altitudes being opposite of what is considered normal because all IFR aircraft are controlled and provided separation. However; I think that this LOA - I could not find the LOA the controller referenced - is a danger to all VFR traffic whether they are on flight following or not. An aircraft on flight following is not required to be given traffic alerts if the controller is saturated and this incident I encountered could have happened whether the oncoming aircraft was on flight following or not; because according to ATC they would have assigned that type of altitude to that aircraft.Some pilots do flight following some days; and other days not. They could one day get an instruction to fly the wrong altitude and be given that LOA information by ATC; and could begin to regularly fly at opposite direction altitude which will increase the chances of a mid-air collision. Ultimately; ATC is utilizing this LOA to reduce the altitude changes that happens at the Florida/Georgia border; which for IFR traffic congestion is sensible; but not for VFR traffic. The amount of risk associated with the LOA applying to their VFR flight following traffic is too high as opposed to them assigning a new altitude for those few VFR flights that cross the border as they continue northeast bound or southeast bound.If one thing could come out of this report to make me feel safe flying cross-country along the east coast; flight following or not; is to get rid of ATC's VFR cruise altitude LOA. The risk associated with wrong altitude flying is too high; and these altitudes take place in some of the busiest airspace in the world with all the airline traffic; VFR personal traffic; as well as training flights at flight schools who are being instructed the proper altitudes but then are given opposite altitudes from ATC. The only reason that this mid-air did not happen is because I heard an alert; saw the traffic; and evaded.