APO DP - saving 'forecast errors' in a key figure

I am using APO DP V5.

When running forecasts, either interactively or in mass processing, I want to store the forecast error (eg MAPE) in a key figure in my planning area. So, for example if I run a forecast in period n, I would store MAPE value in a key figure called for example ZMAPE in period n.

Has anyone done this before?

I note that BADI /SAPAPO/SDP_FCST4 exists. This BADI may be relevant, but I am unclear how it really works. Any ideas on this?

Thanks, Bob Austin