As to how SNP success affects the figures as a whole then it isn't just about how many seats Labour would get. A fall off in SNP would indeed give Labour some more seats but it would also possibly give a potential renewal of the present coalition more seats. The Lib Dems could potentially (according to the polls) lose 10 of their 11 Scottish seats to the SNP. That could be the difference between the coalition gaining another majority or not. So the spin that a vote for SNP is a vote for the Tories doesn't really hold..