Filter → Round Corners rounds all selected corners of a path. The filter is only applied to outwards pointing corners if nothing is selected. The Radius is defined in font units. Choose Visual corrections to balance the rounding of acute and obtuse corners using smaller and larger radii, respectively. Such visual corrections tend to create more natural-looking rounding.
The custom parameter rule is as follows:
RoundCorner; Radius; Visual Corrections
If Radius is positive, it is applied to outwards pointing corners; if it is negative, it is applied to inwards pointing corners. Add multiple RoundCorner custom parameter filters to control both. Set Visual Corrections to 1 to enable it or 0 to disable it. For example, the following rule settings apply a corner radius of 55 to outwards pointing corners with visual corrections enabled:
RoundCorner; 55; 1