How does one go about drawing up perfect gridlines for the ground glass? Do you just draw them parallel to the edges of the glass?

I have this shen hao that I got a couple of months ago, and the lines are not parallel to the edges of the glass, and likewise they don't line up with the film edges. The consequence is that it's a royal pain in the butt to shoot anything that requires tight control of horizontal or vertical lines, including architecture and anything with a straight horizon.

Should I just order a satin snow glass and draw my own lines parallel to the edges of the glass, or is there a more rigorous way to calibrate them (like by burning up polaroids while shooting a grid on paper or something?) What do you guys do? I presume that because the woodwork defines how the film holder sits, you would just have to align the lines with the woodwork (which they are not at the moment).

Should I just order a maxwell screen and get him to draw the lines?