The "lack of consistency" for Warp Factors to actual speeds doesn't make any sense because none of the series has ever put forth a "formula". Without knowing how Warp Factors correspond to speeds in Star Trek there isn't anything we can reference against to call the speeds "inconsistent".
All we really have are occasional distance-time and location data and putting together a picture from that data for the different productions.
Good point. If you exclude the back stage materials, the tech manuals and the semi-official resources in the fan community, Star Trek itself is actually fairly vague on this matter.