If you look at this picture, I want this to be a perfectly rounded corner between these two nodes. I have no idea how far out I have to extent the node curvature pin.
The next method is exact and it works in any vector drawing program which allows rounding a single corner:
The starting situation. You want to change line segment AB to a perfectly fitting circular arc which is tangential with both branches and you are not going to draw it by eyeballing, it must be accurate
draw 2 lines on the branches which end to A and B. Have ON all point and geometry snappings which are needed for exact snapping (different in different programs)
Stretch proportionally (in Illustrator hold Shift as you drag the corner) the new lines so that they cross. You can use scaling to do it and some programs force you to set the line width to zero for proportional scaling because they scale the bounding box
Sent to back the new lines. Move with the node tool node A to the crossing (you must have snap at crossings ON or insert at first a new node to the crossing)
Delete the new lines and node B. Apply corner rounding with the wanted radius.