Capture start and end value of measurement

Hi, I am using an analogue input and a timer to measure pressure decay over time.  I have the live value of pressure on the logo screen.  What I want to do is capture the pressure value at the start of the test and again at the end of the test and calculate P1-P2 to show the difference.

To do this i need to save the analogue input value at the start of the test.   I would use an edge detect to MOVE current value to an analogue flag, but i dont know how to do this in Logo.  Please can someone suggest how I can achieve this?  I have tried using an analogue MUX but get recursion error.

Thank you,

Michael.