How come I am learning about this Space Engine thing just now?
I have to download it and check it out!!!
----------------------
I know that I am trying to build a bridge with bread crumbs, I just though the ME2 ending cut scene partially corroborated the numbers thrown around by Mass Effect Retribution (Using the Diameter and Angle of the milky way in the scene, gave me a result of almost half a million light years of distance, but I rather double check them with the Space Engine program to see if I fucked it up somewhere). The FTL calcs in the OP have several holes and assumptions like no upper speed limit while inside FTL, or no rest for the reapers during the trip, and so on, thus they are very likely to be WRONG, However my plan all along was to use them as a baseline to start discussion and I think I can improve the analysis.
We HAVE variables that are reliable and variables that are not, but a compressible picture can be drawn from them with some effort involved.
For example, when it comes to TIME we have the end of ME1 as the earliest point in the timeline in with the reapers could begin their trip towards the milky way, they COULD have started the trip earlier, but then the whole thing with Nazara/Sovereign and the Heretic Geth stops making sense.
Mechanically we know how fast the reapers are within the stated limitations (Acceleration of 152,082,215.7 meters per second during 24 hours of operation), we know how long "on average" Citadel based FTL drives can operate before reaching charge saturation, and we know most space stations have drive discharge points ships can use meaning the technology to absorb and use the static energy generated by ship based FTL exist and is used, but its somehow too big to use on standard ships or what the ships have can be overwhelmed after a time.
However that is the end of the reliable variables, we have the statement from Mass Effect Retribution that claims the reapers where millions of light years away from the Milky way, however if that was the case then why are they much closer than that in ME2 ending cut scene and why do they appear to have just started moving then? Did they start their journey at the end of ME1 and what we see in ME2 is they restarting their journey after a short rest to their FTL drives? Is everything artistic nonsense? Well yeah, but that is the unfun answer.