put the legs down today. Measured many, screwed once. And ended up not being screwed -> Diagonal was less than 2mm difference. (1.2mm). So over 1.4 meter diagonal I think that is acceptable.
Aha!. spoil board clamping. Well Since the spoil board is 1200x600mm and thus have underlap (under the rails at either end) I am going to drill 2 holes (one each end) 8mm or 10mm (not decided yet) through the spoilboard and board underneath, and use a bolt up from below and wing nut at top (easier to lock down).
Underneath the bolt will be coming up via/through left over bit of 2×4 and recessed in the wood (thus making the wood the securing method). Since the wood is just long enough to fit, it uses the bracing wood not to turn.
Step 2 is future planning – I then take the bolt/wood out, slide the spoil board 1/2 off the edge, and secure the bolt to the board. Slide it back into the table, until the board/bolt is butted hard up against the end. then I drill through the existing hole (now some where in the middle of the table) to make a hole. Repeat the other way.
This Hole I can now re-create via the CNC/drill the next time I have to make a spoil board and have it all exactly lined up.
Still researching what method to secure clamps to the spoil board. possible T-bit via router. or recessed hole/lock nuts. Still reviewing methods and available tools.