For table display to work properly, you really need home switches.
A workaround is to jog to the 0,0 corner of your table, and ref the X and Y axis, to set machine zero there. You also need to configure Soiftlimits, which define the size of the table.
You'll probably need to periodically re Ref the X and Y axis, as they'll move a little each time you turn the machine off.