Storing global values in Gcode (Variable tool length sensor z-position)

I added a tool length sensor to the table of mill to populate the tool length of my BT tool holders.
As the machine does not have encoders and therefor needs to be homed on every start the z-position of the switch tripping point varies on every startup due to the inherent inaccuracy of the z-axis home switch.

My idea is to create a script letting the spindle nose trip the tool length switch and save the value.
Then the value will be used to measure the length of the individual tools.

Does someone has an idea where to store that z-offset so that it is available across Gcode scripts? It does not need to be persisted as the initial value has to be measured on every machine start.

Regards Mark

Numbered is always global, as #111. Named <#local> or #<_global>. #5063 (G38.n) return value. #111 = #5063 or #<_probnose> = #5063, this global parameter can be used in your gcode.

Thanks a lot!

/Mark