VAT charities reverse charging

SUGGESTED

We are a charity and are VAT registered. This means we have to make a partial exemption calculation.  If we buy a software subscription from say the USA, we need to reverse charge this, however we cannot reclaim any of the VAT back if the subscription is attributable to a purely charitable activity. This means on our VAT return we need to include a 20% notional VAT charge in VAT Box 1, with the net amount of the purchase in VAT Box 7. I can't seem to configure any T codes to show produce this result. If I tick the reverse charge box when configuring a T code, then the full amount of the notional VAT always appears both in Box 1 and Box 4, as if it was fully recoverable.  However, as the service being purchased is attributable to a purely charitable purpose, VAT cannot be recovered. 

Has anyone from the charity world managed to configure Sage to produce the right outcome ?     

Thanks