to quantify this, i would start w/ 6 pounds, the reason being, you can scale 6.25 to12.5 to 25 to 50, w/ some minor tweeks, scaling is not strictly additive, anyhow
2oz salt, 6.25 lb shoulder, 2 bunches of onions, or just eye it up, .25 oz crushed red, pinch of thyme, about 20 oz h2o, if you...