Yes that's my point.
5% of the population is huge, really. Because that's a lot much in % of the men. Half of the population is women. For the men, let's say than 50% of them are elder (more that 50 years old), child,
According to the US Census from 1940, all men aged 20-49 (it breaks it down by 5 year groups) comprised ~22.4% of the US population
or people not physicallly able to go in the army.
Unfortunately, the report includes "not able to work" in the same category as "those employed in own home housework, those in school, those unable to work, inmates in penal and mental institutions, and homes for the aged, infirm, and needy."
That's leave us with only 25% of the population being men between 18 and 50. So 5% of the total population means 1 on 5 of all men in age are in the army. In other words that's like sending in the army every man from 25 to 31 years old. And those who are sent first are mostly those who are the most work-capable too, so the burden is even higher.
Not sure I understand your math.
You say that 25% of the population is in the prime service age of 18-50, then say that 1 in 5 of the men in that age bracket would be in the manpower pool. But then you guess that every man aged 25-31 equals 5% of the population?
Rationning and relocating economical ressources by the government is the same as amputating is in surgeonery : a last resort action with dire consequences. Basically it's what the USSR did during its whole existence and they ended with an economy so crippled they even couldn't produce their own food... that kind of trick is not a solution.
The heavy draft was why you started to see more and more women in the workforce during (and leading up to) WWII. Just so you know, roughly 11% of the US population (Male and Female) served in the military during WWII.
EDIT: And that 5% is NOT just 5% of the men.
That is why you can take the 5% conscription level and have varying levels of manpower depending on things (Spirits, Decisions, etc.) like "Women in the Workforce"