As Colin implies, there’s no need for it to be perfectly square, as long as you can adjust it to be parallel to the plane bottom with the lateral adjuster. I don’t like it if the lateral adjuster needs to be to the extreme on one side to be parallel, but I do expect to use some lateral adjustment. What I’m learning as my sharpening skills improve is to carefully and frequently observe the scratch pattern as you sharpen, particularly as you start out. By eye you should be able to see if you are establishing an appropriately “square” bevel.
Also, if you are doing major metal removal to establish the primary bevel, and you’re doing it by hand as opposed to using a grinder, then I find a piece of course sandpaper attached to a flat surface (like a piece of float glass) with spray adhesive cuts a lot faster than any stone or diamond plate I’ve tried. Again, be sure to check the scratch pattern frequently.