The way I grind back rake is to put a line on the grinding rest. It is measured against the platen of the grinder, away from me and to the right of the belt. Align the tool with the angle on the table, press into the belt.
If the tool side is facing the camera, I think you are going the right way. If it is your first tool, the high angle is probably not the best choice. Start with a lighter angle like 15 degrees. It will still cut well on aluminum.