Can somebody please help me with this strange (551mm) size that I get when I want to setup a fixed extension of Top rail of railing at bottom of stair?
I have setup a "300mm + Tread depth" extension for railing at bottom of stair but I get this strange 551mm instead of (580). I noticed that removing "Start post" from Balusters list in railing setting will fix this but I need to have this first post.
I have snippets of the settings and floor plan.



