Is the stamp duty contingency principle relevant to a share transfer where the consideration is adjusted by reference to completion accounts?

As detailed in Practice Notes: Stamp duty on transfers—consideration and calculation and Stamp duty and SDRT on the sale of certificated registered UK shares (see the main section ‘Cash consideration determined by reference to completion accounts’):

1. •

   for stamp duty, consideration must be ascertainable (ie capable of being calculated) at the date that the instrument of transfer is executed—if the consideration is truly unascertainable (which is very rare), there is no value by reference to which stamp duty can be charged

2. •

   the contingency principle extends the meaning of what is ascertainable for stamp duty purposes

3. •

   the contingency principle applies where, at the date of the instrument, the amount of consideration is uncertain because it depends on a future event occurring

4. •

   the contingency principle calculates stamp duty by reference to any maximum sum (this includes where a definite maximum consideration is stated or can be calculated from the document), even if that sum is variable or may never become payable

5. •

   where