As you probably saw on last night's post, John Curtice made clear that so-called "tactical voting on the list" becomes very risky as soon as you assume (as you really ought to assume if you haven't shut down your brain under instructions from a RISE press release) that there is a chance the election result may differ somewhat from current opinion polls. Crucially, the direction in which the polls lead us astray doesn't really matter - if the SNP are being overestimated by the current polls, they'll need list votes and list seats simply to retain their majority, but if they're being underestimated on the list vote, they stand to win a decent number of list seats even if they take a clean sweep of constituency seats. Both of those possibilities are very real, but if you were a gambling man/woman, you'd probably be betting more on the latter, simply on the basis of past history. And if that's how it works out, SNP supporters switching "tactically" to a fringe party on the list could easily reduce the pro-independence majority in the Scottish Parliament.
Although the SNP's strength on the list vote was very severely underestimated in 2011, that doesn't always happen - in fact, their list support was overestimated a tad in 2007. But there is one very clear pattern that was seen in both elections, namely that the difference between the SNP's constituency and list vote was overestimated. If you look at Wikipedia's list of 2011 polls (which may not be completely exhaustive, but it's the best one I can find), an average of the final eight polls suggests that the SNP should have done about 4.1% worse on the list ballot than on the constituency ballot. The actual gap on election day was just 1.4%. The story in 2007 was very similar - the last eight polls listed by Wikipedia suggest that the SNP should have done 3.5% worse on the list than in the constituencies, but in fact the real gap was 1.9%.
Yet again, the polls this year are "predicting" (to misuse the Sunday Herald's favourite word) an implausibly wide gap between the SNP's support on each of the ballots. Every piece of logic would suggest that they're probably wrong about that, if nothing else. So in trying to interpret what the polls are really telling us about the current state of play on the list, there are two basic options - either we can assume that the polls are broadly right about the constituency vote but are underestimating the SNP on the list, or that the polls may be somewhat wrong about both ballots. (Unless it's by complete chance, they're unlikely to be right about the list and wrong about the constituencies, simply because the constituency question is asked first,) No matter which of those options you favour, the tactical voting lobby are left with a big problem. If we can't entirely trust what the polls are saying on either ballot, then all of the incredibly precise "predictions" (ie. projections) of seat numbers that are being used to make the case for tactical voting are completely meaningless. But if the SNP are being underestimated on the list, and can actually expect to take around 50% or more of the list vote, the projected number of list seats for the party needs to be adjusted upwards.
Take the new Panelbase poll, for example. If you pump the headline figures into the Scotland Votes calculator, the Greens take nine list seats, the SNP take five list seats, RISE take zero and Solidarity take zero. But if you make a very modest adjustment to take account of the polling error that was seen in both 2007 and 2011 (ie. if you increase the SNP list vote by 2% and decrease the Green list vote by 2%), all of a sudden the SNP are left with more list seats than the Greens - in spite of sweeping to victory in 66 of the 73 constituency seats. In that scenario, Green list votes would be "wasted" in slightly more regions than SNP list votes (and of course RISE and Solidarity list votes would be wasted everywhere). And that's before you even take into account the fact that SNP list votes will be an absolutely vital safety net in the event that they do just slightly worse in the constituencies than this projection supposes - if they take two fewer constituency seats, they would need at least one list seat to retain their overall majority.
So it's categorically not the case (as Tommy Sheridan said to me in our debate a few weeks ago) that only "pessimism" about the SNP's prospects could possibly lead us to think that SNP list votes will not be "wasted". In fact, even the most conservative optimism about the SNP's prospects on the list vote would mean that we might expect the party to win more list seats than all of the smaller pro-independence parties combined, regardless of the results in the constituencies.