Manual tool change + tool probe

Hello,
(I don't speak English very well, I hope you will forgive me)
I have just finished building a mini CNC with LinuxCNC.
I use a BOB for parallel port and the Gmoccapy user interface.
I attached a push button to my board to be used as a tool length measurement probe (I know it won't be accurate, but waiting for better).
My spindle uses an ER11 tool holder. For the moment I don't mind changing tools by hand.

I followed the instructions on this page to get the manual tool change from Gmoccapy and remap M6 to take a measurement at each tool change:
linuxcnc.org/docs/html/gui/gmoccapy.html...uto-tool-measurement

But this procedure doesn't seem to fit my use. I tried to change some settings in the .NGC file of the subroutine, but without success.
Here is how I would like to use my machine:
- mount any tool or keep the one already installed
- jog exactly where I want to start my machining on my block (on top)
- do the zero part (X - Y - Z)
- start my program

I would like all following tool changes to apply the offset according to the first tool (which was used for the part origin).

So I would need some guidance in modifying the tool change and tool measurement routine.

Thanks in advance

Thank you for this answer.
I had read the documentation on G codes, but I didn't fully understand the difference between the different variants of G10.
I understood that some act on the part origin and others on the tool table, but the details and nuances I did not understand.