But the simplest method is to ensure that the to be processed edge/vert’s and the mesh centre line of the other two axis align exactly on the X-0 Y-0 Z-0 so that when you pass your pointer over the edge to position the blue plane, there is no need to take note of the xyz stats’ nor input them negatively, (but keep a beady eye on them), because all are at or should be, zero and hence after applying symmetry ready to average weld.
That works well if you’re trying to make a complete object from half of one, but Ausair’s problem is more along the lines of having the right-hand wheel of a car and wanting to create the left-hand wheel.
To have a distance between the symmetry, you need to change the options…[to enable]... the “Center of symmetry”
This is one of hexagon’s famously frustrating hair pullers.
The “center of symmetry” option would be an outstanding option except that its knees are broken by the lack of precision. There’s no way of assuring an X-value of 0 while mirroring across the Y-axis. Hexagon, much as I love it, is about the only 3D app on the planet that lacks this no-brainer ability. It works okay as an eyeball-estimation tool, but once the mirrored item is placed you still end up having to tweak its location by inputting the X value.
Maybe it’ll get fixed in Ver. 3…