AHHa ! Was just about to ask if there were examples of TLO switches in emc. A member here named Bubba (I think) has a blog with a Mach example.
To the original question, zero can be punched in as "touch off" if the machine is at the intended x0,y0. If using a pin or edgefinder, punch in the measuring tool radius +/- accordingly.
In axis, click on Machine->show emc status
This will pull up some useful information on position. Somewhere in your emc folder the is a file called stepper.var. Play around with the touch off and see how the lines 5221, 5222, and 5223 in stepper.var match the touch off position. Line 5220 needs to be 1.000000. These are the same lines used for offset G54 btw.
G54 thru G59 is the way I'll be programming as soon as my home switches are setup. Line 5220 in stepper.var can be 1.0000, which corresponds to G54. A 2.0000 would activate G55 which uses the lines below 5223 etc.
Just a bit of FYI. Hope it helps in someway.
Anyone who says "It only goes together one way" has no imagination.