The reality is sometimes everything is not concentric.
Sometimes the tang hole or bushing may have a few thousands of slop.
Last couple I made did not, but usually they have a couple of thousands off somewhere along the path of the bushing and tang interface.
That is why you make an index mark on the bushing, I just scribe a line at 6 O'clock to be sure what I see at the back spring and liners is always the same.
This is how I measure if there is any slop with the bushing and tang hole.
On my rise fall/Rupple Jig indicator I place the bushing, blade and spring.
I zero the indicator on the open position.
Move the blade to half stop allowing the bushing to rotate with the blade.
Hold the bushing down and do not let it move, then move the blade back to open.
Look at the indicator, did the reading change from zero?
Now repeat, moving the blade to close, allowing the bushing to pivont on pin.
Hold bushing down and move the blade all the way back to open.
Did the indicator return to zero?
If there is a change in the reading I rotate the bushing, on the indicator to the lowest reading point below zero. This way if the bushing moves it will only move the spring up and not down into the liners upon completion.
Hope that makes sense. I have not seen anyone else do this. It is something I worked out