1 on map MP = 10,000 people
Province A has 1,000,000 inhabitants = 100MP.
Province A is part of country C, in with mediocre agricultural techs and established standards (à la Vicky-mechanized-mining-events) of the region (B) it is part of require 15% of the population to work in the field and food processing sector, in order to feed the other 85%.
Further MP is deduced for ressource gathering operations, numbers depending on amount of ressources and techs and regional standards, again. Say another 15% for the case of A.
So, on the provincial level, we come out at 70MP. This number gets combined with the those of the other provinces of B, as an aggregate of the regional MP, from which IC requirements are then deduced. B has 4 provinces, all like A, making for 280 MP at this point, and 20 IC, requiring 10MP each to run perfectly: 280 - 200 = 80... 80 / 4 = 20. In other words: Each of the four provinces has 20MP left from its 100MP base.
The sum of regions is taken and devided by the total raw MP (those 100MP of A and all other provinces) to find the avaiable percentage. If it is lower than what the current drafting law allows, it is the number of avaiable MP to be displayed - if it is higher, than the raw-MP-sum * current max-draft rate is (law acts as a cap to maximum).
The number can be pushed below zero - that´s when it starts to hurt your economy. When it does get negative, MP is taken from the industry sector first hurting production effeciency (nationwide should suffice), until some ressource linked conditions prevail for some time (surpluses and stocks), resulting in MP being taken from ressource gathering, hurting that. As last resort, if everything else is already strapped, MP will taken from the agricultural sector, but that can only be sporadic, as it sends dissent through the roof in no time. Some algorythm takes care of from which sector to take how many MPs, along the rough lines just layed out.
This makes any collapse because of manpower issues more like wading through quicksand, rather than hitting a brick wall.
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Edit - the same stuff in other words, mostly:
4 sectors, in this order: Agriculture, Ressources (maybe subdevided), Industry, Military. Each MP is either employed in one of these, or in the pool (which models not only unemployed but also, well, let´s say, non-essential jobs). Each sector has an effeciency based on how ´fully stocked´ with MP it is. If the pool - whose sum with the military sector is capped by the draft law in relation to your total population - hits zero, the game will start taking MP from the other sectors.
The player should have some control over the sector it takes the MP from (thinking of priorities like those in the espionage tab of HoI3), but it should work automatically mostly.