Win32 scrollbar “slop distance” constant?

On Windows, when you use the mouse to click and drag in a scrollbar to scroll it up and down, you can move the mouse approximately ~120 pixels away from the scrollbar before the dragging stops working. (By contrast, on the mac you appear to be able to move it anywhere on the screen and the drag will still happen.)

Is there a constant I can get at, maybe using SystemParametersInfo or something, to get the OS-defined value for this? I'm using a control that simulates a scrollbar, and I'd like to use the same behavior for how far you can drag away from the bar before scrolling stops.

Thanks!