Not sure whether you are seriously asking about how to set a 45-deg angle or not, but I'll assume you are.
It doesn't matter. You aren't trying to assess how much the focus is off, only that it is off. The key points are to make sure the camera is focussing on a completely flat target and that the angled ruler does not enter the focus area at all. If this happens, the camera might legitimately pick part of the ruler away from the intended point of focus. That's why all those picture aimed at the 6 inch mark of a ruler or complete cobblers.
Before pfaffing around with a complicated set-up, you might like to read this article. http://www.rogercavanagh.com/helpinfo/23_focuscheck.htm, which describes how Canon Germany say to test focus. It's a lot less hassle to set up, although it will only tell you whether focus is off, not if you are getting front or back focus. But off is off, and if you do need to have the camera calibrated I don't think the Canon service technicians will care what you found.