What you are proposing doing is fairly standard; chose a location that stays there and touch off from that, then have a value for Z in the G54 or whatever work coordinate you are using.
This has been discussed before and my contribution has always been:
Make your touch off position higher than your highest job. This way your Z values in the work coordinate a negative because you have to come down to the job. The reason I suggest doing it this way is that if a mistake is made and a positive value is entered then the tool goes too high and nothing interesting happens.
If you make you touch off point lower than the job then the Z value has to be positive. If a mistake is made and a negative value is entered then your tool goes tool low and lots of interesting things might happen.