Pretty easy, touch off your vacuum table with any tool (pick on that is easy to get a good height with or your flycutter). Enter that absolute value as your tool offset for that tool. Then in the offset screen, make your z offset equal to the thickness of your spoil board. You may need to start at the nominal thickness of the spoil board and move down (by reducing the z offset) until it's flat. After that, you can touch the remaining tools off the spoil board (but then you need to minus the z offset from the absolute value).
To resurface the spoil board, reduce the offset amount only by say .5mm or 1mm or whatever and do a fly cutter of the table at 0 depth (in Vcarve).
For example, my grid top table is say -369.02mm for my. 25" cutter.
My spoil board is currently 8mm (started at about 16mm). If I touch off the top of the spoil board right now, I should get -361.02 then I minus 8 for the spoil board and enter - 369.02 in the tool offset. When first had a new spoil board it would have been top of spoil board -16 (or whatever based on the original thickness).
Don't worry if you absolute value is way different than mine (I might have screwed it up when I first got it), these are all about the relative spacing.
Hope that is clear as mud (at least when you have it in house).
I had a touch off but it was not level and had some give on the mounting so I do it manually anyhow.
Sent from my LG-D852 using Tapatalk