it doesn't physically control the screw, you set your setting on the stove to maximum stroke (most stoves, screw all the way in), then you can select thru the CoalTrol keypad Thermostat the MIN and MAX feed rates (0-100) to match your stove requirements.
Every stove has different stroke rates to control the idle and max burn rates. My Hyfire runs about MIN 6 and 48 Max without pushing hot coals off the end or going out on idel. You may have to tweek the adjustments (up/down) to maximize your stove requirements. But initial setup and adjustment, it maintains the stove output where you set your thermostat.
I have been very happy with my upgraded stove to the Coal Trol, it only had speed (slow/fast) for stoker/fan rates. No thermostat or anything. Just speed it up for more heat or slow it down for less heat, I needed something automatic.