I have two 'L' brackets, one small and one larger. They were cut on the CNC and holes bored for dowels on both the brackets and spoilboard so I can take them on/off and know they're in perfect registration each time.
When I started using the CNC I located to the left bottom corner but have switched to almost entirely the center of the piece to be cut. Like Oliver, I find it much easier to locate the center 'X' than to hit the corner precisely.
For mounting I just use drywall screws directly into the spoilboard but occasionally use small homemade clamps. When the screws are flush with the top of the work piece I never have to be concerned with hitting a clamp with the bit.
I home the machine each time I start it and X0 Y0 is different for each job so it wouldn't do me much good to have a fixed position for that. I also move pieces all over the table to help distribute wear on the moving parts.
Here are the simple L brackets with dowels and a piece I'll cut later today with the center already marked. The brackets are not fastened or pressed down when this photo was taken. Attachment 359491