At the 390BC Battle or Allia, Rome fielded 4 legions when the Republic wasn't much more than Rome and surrounding area. I would think we can assume those 4 legions aren't the total military output. If we assume the max is 8 legions instead, and conservatively put the republic at 1 million citizens, that's 2.4% of the population in the legions.

What are you talking about? When you imagine the Republican state at 1 million citizens, are you talking about the state at the time of the Allia or later, when the state was...well, an actual state? In either case your figure is grossly in excess--the city of Rome itself didn't have a million inhabitants during any period of the Republic, much less a million citizens. At the time of the Alia Rome's population was probably little more than a couple hundred thousand at most. Athens, a vastly larger and more opulent city during the 4th Century, had a population of no more than two or three thousand, of which maybe a third were citizens. Athens could barely field 20,000 hoplites, apart from the rowers in the fleet--Rome c.390 could not possibly have fielded much more than that, even with their allies.

Your calculation of the legions is also very much erroneous, and again I'm not really sure what time period you're asking about, the Republican state lasted for several centuries. The legions of the early Roman state were composed of those landholders who assessed enough to equip themselves for combat. We don't know how large legions during the very early part of the Republic were, but the Roman army cannot have been much larger than the three legions of the kingship, one per tribe and numbering only a thousand men--the consular army of two, later four, 6,000 man legions is a much later innovation. Your estimate of the "maximum" (if we can really talk about such a thing--the Punic Wars showed that the Republican state was perfectly capable of and willing to violate the normal procedures in order to increase manpower) would depend entirely on the period. The consular armies would've been it, period, at the Allia, but following Sulla's legislation you'd see dozens of little four-legion armies popping up in every single province--after Actium Octavian was left holding a bag of over 60.

As sort of a final note I'd really question the value of comparing the manpower of the Roman army at the Allia (to the extent that we really know it) and in the later Empire. We're talking about a difference of hundreds of years, during which time Roman society completely changed. They're not similar enough to really be a useful comparison at all