Looking at the lower Tech Levels, from 1 to 4, forget things like Resource Units, as at that level, it is strictly dependent on population. A really hard-nosed militaristic ruler might get away with 5 per cent of the population in military units in peacetime, but he is hurting his economy by doing so. Frederick the Great managed almost 10 per cent of his Prussian population in peacetime, but roughly half of his troops came from somewhere besides Prussian, which led to his Non-Coms raising prevention of desertion to a fine art, and he had a very long list of exempt occupations who were not to be enlisted under any circumstances. Ten per cent is the max you can expect from a lower Tech Level, as long as you do not have too much cavalry or equivalent animal-using units. Horses or the equivalent are expensive, not just to breed and train, but to feed. Ten per cent is borderline breaking your economy, as you do need people producing the necessary food for all of your troops, and at Tech Level 4 and lower, that is not the easiest thing to do. More than 10 per cent, and you are breaking your economy, war weariness is going to set in pretty fast, and you will be the ex-government, and likely buried.
At the lower Tech Levels, do not think that you are going to put all of your manpower is one spot. At Tech Level 4, you do have railroads, so can get away with concentrating 100,000 men or so at one spot. Try that at a lower Tech Level, or at Tech Level 4 without access to a railhead, large navigatable river, or well-equipped ocean port, and your 100,000 man force is going to be starving in short order. Animal-powered transport is a totally different subject for discussion. You want to get an idea on how hard it might be to supply a Tech Level 3 army, read about the experiences of the French in Spain from 1809 to 1814. The Spanish guerillas feasted figuratively and literally on French supply columns. Read about the problems the Union Army had at Chattanooga following the defeat at the Battle of Chickamauga.
And an aside for supplying horses with food. Thirty horses will eat an acre of grass a day in good pasture land, if subsisting strictly on grass. Feed it 10 to 12 pounds of oats a day, and the acre will last 3 days. However, you do have to grow the oats somewhere, and then get them to the horses. There are 32 pounds of oats in a bushel, and Garnet Wolseley has oats raised in England at 40 to 50 bushels an acre, being a cold-hearted Quartermaster Officer, I would hesitate expecting more than 25. Regardless, your 30 horses are eating about 10 bushels, or 320 pounds of oats a day, every day. You have to deliver that to them, possibly using other horses, also eating oats, every day. Your 100,000 men are also eating, about 3 pounds of food per day, so 300,000 pounds or 150 short tons, every day, needs to be delivered. Then there is the matter of water for everyone, a minimum of one gallon per day per man for drinking, and about 6 gallons per day per horse for drinking. Always have to remember water supply along with food. Men and horse die of thirst before they die from hunger.
And if you think that is bad, an Air Force is much, much worse. A base figure for a 75 aircraft wing is 5,000 men directly supporting the aircraft, without about the same number in indirect support. Using current jet aircraft as a basis, your 5000 MCr Resource Unit might, repeat might, support 250 single-seat jets. It will not support more than that.
To get some ideas on how different types of governments like to spend money on defense, check out the CIA World Factbook. Germany is currently spending less that 2 per cent of its GNP on defense. Using that as an example, to get 1 Resource Unit for defense, you need the planet producing 50. Most countries, unless involved in a direct shooting war, do not like to spend money on defense.
