Im not sure what 3.1/2 12 strand is, but it sounds a bit over kill, the twine sounds thick and the mesh size maybe a tad small.
When I used sheet netting,(6z) I just cut to lenth and put a running line on, I left the kill as it was, which was fine. The mesh I bought had doubled up edges, effectively a built in selvedge.
On the one I`m making from scratch, I`m using a 2-1/8 mesh board, which will gimme a 4-1/4 mesh. It will be 14 meshes deep and I`ll be adding a selvedge mesh top and bottom, making it 16 meshes deep. Your net will work without a selvedge mesh, but over time and use, your lin