Seasonal forecast calculation details

Hi,

I'm using transaction MM02 to calculate seasonal forecast ("S" model", with no optimization marked);

I entered 18 months consumption and get a correct forecast for the next 18 months.

Thanks to the formula described in HELP.SAP,  I tried to reproduce the calculations in excel, but unsuccessfully. The main question is : what is "V(t)", actually.

Can someone send me the correct formula for G(t), and tell me what "V(t)" is ?

Monthly consumptions, calculated forecast and used parameters are in the attached file.

Thanks a lot in advance,

Sandrine