I have a grid on my spoil boards that I burnt with my laser but a shallow cut with a V-bit works fine as well. As far as the placement I centered mine. The problem, as I see it, with basing the grid on the front left corner is that you can’t clamp anything at [X0, Y0] and have room to use a touch block.
If you go with it centered just measure the work area to find the center, you can use gSender and measure with the mill, and start the carve from the center. If you do go with the front left I would leave room for at least using a touch block and would think about starting it far enough from the edge to make a removable corner fence to make mounting work easier. With a nice fence and some toe clamps you can mill the whole top of a board at once with no clamps in the way.
I have posted a couple of spoil board grids in my Dovetail Guide, the last half of the thread, if that helps at all. The pics are svg, right click->save as, I also posted crv files if you have Vectric.
I also wonder what you mean by absolute point? Do you mean measuring or the small machine coordinates in gSender. If your talking about the machine coordinates, it depends on whether you home the machine or not. It’s been awhile since I ran without sensors for homing but I think machine [0, 0, 0] is wherever the machine was when it was turned on without homing.
This other post of mine explains how I use my centered grid for double sided work if your interested.