It does work for me? Are you setting your symmetry the same place you set your helper, when you place the helper?
Your response makes me realize that I did not explain myself sufficiently.
So let's say you lay some girders starting at the ship core leading down negative Z for 20 blocks. Now select YZ symmetry on that girder, and place a girder from the end of the first along X both positive and negative for 10 blocks. Then open the Build Helper pane and select the end of one of those girders on X, either one, and create a circle of radius 40 with Y rotation of 90 degrees. This will plan a circle in the YZ plane parallel to the central girder and offset from center by 10 blocks.
So you can draw the circle on your build helper, and symmetry will give you a second circle mirrored across the central girder. But if you select Restrict Building to Helper, then the only blocks drawn will be where the helper is highlighting, and symmetry is ignored. You only get one circle. To get its mirror, you either need to recenter the helper at the end of the other girder or deselect Restrict Building to Helper. And either way, you need to build some scaffolding to reach the first block of the helper region.
So here is my problem. Circles have a step offset for new portions of the arc, such that parts of the circle meet at the corner edges of blocks rather than at the faces, so to draw the highlighted helper block you must first place a block that is not on the circle, because a block cannot be placed in empty space without scaffolding. Blocks cannot be placed against corner edges, but only against faces. For this reason, when using the helper (particularly for large circles, say larger than radius 50) I set my brush to a 10x10 plane, select Restrict Building to Helper, and paint along the helper so that I drop about ten blocks at a time and do not need to worry about scaffolding. My issue is that if I do this, symmetry is ignored.
The essence of my problem is that symmetry does not affect the Build Helper mask. If the symmetry tool created additional masks, that would great.
