Underdrawing a bow makes it essentially worthless, so we don’t represent it. If you take a typical-ish hunting bow at about 70lb peak draw and only draw until it reaches (let’s make the numbers simple, half weight) 35lbs, you aren’t imparting half the force, you’re imparting something like a quarter of the force, since bow power is very roughly “peak draw weight * draw distance / 2”. If you draw a 70lb bow to 35lbs, you’re cutting both the peak force and the draw distance in half, so to plug in some real numbers (28" is a very standard draw length):
70 * 28 / 2 = 980
35 * 14 / 2 = 245
If instead you have a bow designed to be drawn to 35lbs, it is about half the power of a 70lb bow.
In other words, a 40lb draw weight bow delivers roughly twice the force as a 70lb bow underdrawn to 40lbs.
If anything, this simplification greatly underestimates the impact of underdrawing a bow, due to various design issues, the vast majority of the work is done in the last couple of inches of bow travel.
That 70lb peak draw weight bow is considered “enough to reliably take down a deer with a square and well aimed hit”, so drastically reducing it gets you essentially a toy.
OTOH, as @Valase points out, a 40lb peak draw bow is considered barely enough to take down a deer with an essentially perfect hit, from close range, on target in the vitals, from a good angle, and avoiding any bones.
We have some amount of interest in specifically allowing for the player to work their way up to higher effective draw strengths, it’s a top contender for special purpose strength training, but we’re not there yet.