The drain flow rate for a 10" softener will be typically 2.4-2.5 GPM to sufficiently lift and expand the resin across the entire 10" cross-section.
The flow to the drain is 'powered' by the incoming water pressure. If the well pump's pressure switch setting is 20/40, 30/50 or 40/60, that will be the pressure supplied to the DLFC (drain line flow control = flow restrictor).
Although water can be pushed virtually any distance, the flow rate needed to backwash the resin is specific. Because a long length of tubing may cause friction that can reduce the flow rate, the usual recommendation is to increase the drain line diameter when the length will exceed 20' or the drain line is to be routed up higher than the softener. This is intended to ensure the required drain flow rate is maintained throughout the entire drain line length.