So here's the problem I am having

I am trying to calculate a flow/purge rate from water being purged from a well

10:43am static water level (SWL) was 7.82m known purge rate of 8LPM (litres per minute)

10:46am SWL was 7.87m purge rate ?

10:49am SWL was at 7.9m, purge rate ?

10:52am SWL was at 7.925m, purge rate ?

10:55am SWL was at 7.962m, purge rate ?

10:59am SWL was at 8m, purge rate ?

What I would like to do is have a formula that automatically calculates the change in purge rate. Any information would be greatly appreciated