How to 'Snap' the UV map with Mirror Modifier

I dont know how to be precise in UV Editor with a texture mirror by a mirror modifier.
‘modified’ is check in the view menu (green)
How can I found the right ‘V Offset’ number (green : -0.4227) to have the exact opposite ?

I dont understand where is the origin of the mesh in the UVMap…
UVmap_Mirror.blend (638.1 KB)
thnx for your help