Installing vespatronic into Vespa 90 - initial stator setting?

I have installed a vespatronic into my 90. 

SIP has told me to set timing at 25 before TDC.

Can someone help we the initial setting of the stator when it is installed?

As I have it now when I try to start all I get is backfires.

So if I can't get it to start then I can't set the proper timing.

by the way I have there tuning kit which includes the Malossi 136 cylinder and Dellorto 25mm carburetor.

thanks!