Well, that video would not load for me, so here's my version...
After I installed the table on my machine, I ran a routine to machine the table flat. So when I attach a piece of material to my table top, and that material is smaller than the flattened portion of that table top, I know my machine can reach any part of that material.
So with that in mind, here is my process.
1. I will typically design with zero in the center of the work piece, and on top of the material.
2. Attach material to table on flattened portion of table.
3. Hit "ref all home", your machine should travel to its home limit switches and set the machine coordinates to 0-0-0. There should be a button under your digital read outs (DRO's) on your Mach 3 screen that is labeled "machine coordinates" - when you click on this (and the little red indicator light is lit), the DRO's should read zero for x and zero for y.
4. Move the machine so that the cutting bit is in the center of the material. Write down the "machine coordinates" of this location in case you have to reestablish the center.
5. Zero your machine to the top of the material, and click on the "machine coordinates" button again (so that the red indicator light is not lit), so that you can see the table coordinates. Zero the table coordinates by clicking on the x,y, and z buttons beside the DRO's.
6. Load your g-code.
7. Turn on router.
8. Press start.
9. I keep my finger near the stop button while the machine is running.