Touch Probe and IRC5 Controller

I have an ABB120 and IRC5 Controller. I have a known position and it's Z value (call this 0) and i want to approach this position from say 10mm back and when I see the input from the touch probe go high, i want to know the distance from my know point to the touch point to measure the size of an object picked up by the end of arm tooling. How would you guys approach the problem of collecting the distances ?