Whilst designing my own CNC from scratch after being hooked by the mostly printed CNC I decided one thing I had to have was a method to fine adjust the bed work area other than the legs. As can be seen from the photo I have used 40mm M8 and plenty of adjustment available board pictured is 18mm MDF.
So here is my simple solution the bed can be adusted at 4 corners and if necessary middles too by use of top and bottom m8 nut and bolt.
Simple but effective I will be combining and incorporating into the feet of MY MP_CNC in the future when time allows and will upload that version when done.
Slack off both screws and level by adjusting bottom screw before locking in place with top screw.
As many or as few as required can be used on larger beds one or more adjusters could be used along each side to take out warping.