Having seen and read a lot, I'd set the following bars:
SNP; winning the seat would be a fantastic result and a political earthquake. Coming within 2000 votes would be good, 3000-2000 Labour majority fine. If Labour hold on by more then there may be some sould searching.
Conservatives: They start 4th with only 7% of the vote and should be squeezed in classic by-election style. The Tories should be happy with 6-8% of the vote and more than 10% of even coming third would be an excellent result.
LibDems: Once again there has been lots of LibDem ramping. An utter disaster would be coming 4th, an OK result would be holing their share of the vote and any increase can be spun as good.
Labour: A tough one; a low turnout will cut Labour's majority but they must win here by more than 3,000 or so for them to have a good result. A three figure majority or less will be poor for the party and losing it would be a disaster.
We'll see what happens . . .