Both my Classic and Retail installations use the following setting:
2560×1600 px, UI scale 0.64 according to Elvui, uiScale “0.64999997615814” according to CVar (config.wtf). When I change Elvui scale it does not change config.wtf, but Elvui states that the setting is per user and I did not find the corresponding per user settings file yet.
UI scale does affect small window placement, so there could be a rounding issue. For comparison: Kaliel’s tracker does not move relative to the minimap when UI scale is changed, so there are different implementations.