Could the % top slice rate be tapered down for large contracts (or increased for small contracts?)

Yes, the consortium board, accountable to the wider membership, would have the option of reducing the % top slice if large scale contracts were secured and, feasibly, it could be as low as 2% or even 1% in such circumstances.  The top slice rate is merely a device to give the consortium enough money to pay for essential secretariat and development functions (and nothing beyond that).  The consortium will need to ‘future-proof’ itself by building up a small amount of reserves in line with Charity Commission regulations but otherwise the entire focus of the consortium will be to cascade any resources secured to local voluntary organisations to enable them to deliver vital frontline services.

By the same token the board would also have the option of, in exceptional circumstances, increasing the top slice rate above an anticipated maximum of 10%, but a special case would have to be presented to support this.