The Electoral Reform Society rules for running an STV election includes the following:
"5.4.8 If any papers have become non-transferable before any candidate has been deemed elected, recalculate the quota as in paragraph 5.1.6, ignoring the non-transferable vote."
which equates in my mind to removing spoilt ballots?
It doesn't mention any further quota recalculation, in the preface it states
"The opportunity has been taken to simplify the structure of this publication, to clarify its wording and to remove some duplication. The principal changes are:
Withdrawal of the optional provision to place votes in suspense, which was introduced in the second edition. This appears to have been little used. We believe that the increased use of computers makes it unnecessary
A further reduction in the quota when it is safe to do so
All quotas calculated to two decimal places."
So presumably in the ERS' view, a reduction at any other point is not safe?
I think one effect of varying the quota at multiple points would be that the vote totals would vary - at present, if all the remaining active votes and the total non-transferable votes are tallied at each stage, you should have exactly the number of ballots at the start.