Manual home to a specific location

I would like to find out how to tell LinuxCNC in absolute terms that your position is x,y,z.
Not relative. I want to set the actual position of x,y.z.
I do not have limit switches to home.
I manually home to a custom jig at the center of the machine.
After I do this, I need to tell LinuxCNC "you are here".
Everything I've tried creates a corresponding negative relative offset.
Please help, and thanks in advance.
--LoneWolf.Quest