You can do it all on the offset page.
Touch your probe off on the + side in X and with the cursor on the X column on a line for a work offset you are not using press Zero Part Set.
Go to the - side and touch the probe off, move the curtsor down one line in the X column and again press Zero Part Set.
Use your calculator to take the average of the two X positions and enter that into the X for the work offset you want to use.
Do the same for Y then just wipe out your entries by entering 0.0 and pressing F1.
This way you do not need to know the diameter of your probe, and if you rotate the spindle 180 degrees between each side so you are using the same location on the probe it does not even need to be running true. And if you are using paper for touching off the probe you don't have to worry about the thickness.
An open mind is a virtue...so long as all the common sense has not leaked out.