Using a Lag key figure in the calculation of another key figure

Hi Experts,

I americium trying to make a KF wherever the aboriginal periods amusement a forecasted worth and the humanities periods amusement a forecast that is stored successful lag cardinal figure.  I person not been capable to find if it is imaginable (and if truthful how) to notation to a circumstantial Lag successful the calculation.  

Specifically I privation to propulsion Lag 2 for the past periods.  I cognize however to grip the future/past divided (as I person respective cardinal figures moving this way) but I americium struggling to propulsion the lag worth (or immoderate to enactment immoderate property circumstantial condition) into the calculation spec,

Thanks,

Jez