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http://www.bbc.co.uk/news/election-2015-32601281
This gives the split under the D'Hondt PR system (which assigns seats using PR in regions, so Scotland is handily self contained). So the SNP would have got 31 seats rather than 56, just over half which isn't bad given their poll (thanks to your figures) of 49.97% of Scottish votes.
More opposition and a fragile majority.
Greenbank-
I'm not hugely in favor of list based PR on the whole, mainly as I like the idea that MP's are tied to places, it makes it harder for them to tell you to get knotted. Even region based D'Hondt isn't quite close enough for me for this, as the areas are too large. It also prevents independent candidates from standing.
I think that personally I'd lean towards larger constituencies, with a number of top-up MP's that are based on the percentage of votes cast (maybe using a regional D'Hondt system) much like the London Assembly is, but I'd be happy to go with something else if I could be persuaded of its merits. I'm also not sure if the top-ups should be on the basis of the initial vote, or a separate one, as I could be persuaded to vote for an excellent constituency MP, but not like their party. Also see the independent candidates point above (I think that I've just explained that I'd rather it was separate).
MrDrem
So, I did the calculations that I wanted the electoral data set for.
Essentially I'm fed up with the argument that the SNP don't 'deserve' their 56 MPs as on a national level they only got 4.7% of the vote, and that's unfair on UKIP (etc) who got 12.6% of the vote and one seat.
In essence FPTP and the deposit system should encourage parties to only stand in seats that they think that they have a chance of actually winning. The SNP did this far more effectively than UKIP, essentially taking a rifle to the deer hunt, rather than a shotgun. This is not a defence of FPTP, but part of how it works.
UKIP stood in 624 seats, that had 44,684,664 registered voters in them. Of those voters, 29,528,063 bothered to turn up to the polls. UKIP won 3,881,099 of those votes. This gave them the votes of 8.69% of the electorate that could vote for them, and the votes of 13.14% of the electorate that bothered to vote in those areas.
The SNP stood in 59 seats, that had 4,094,784 registered voters in them. Of those voters, 2,910,465 bothered to turn up to the polls. The SNP won 1,454,436 of those votes. This gave them the votes of 35.52% of the electorate that could vote for them, and the votes of 49.97% of the electorate that bothered to vote in those areas.
Both of those percentages for the SNP are, incidentally, higher than for any other party.
Data for this is here: https://goo.gl/PLLTKa
Oh, and if you want a cycling analogy for it, it's close to the one about red light jumpers. The percentage of cyclists and drivers that both have the opportunity to, and that do jump red lights is similar, but people think that more cyclists do it because they see more of them, because they have more opportunity to do so. The first car that stops at the lights prevents all the cars behind them from jumping them, the first cyclist there doesn't prevent the second or third or fourth from doing so.
This is why it's really important to ensure that you're comparing the correct things, as manipulation of stats happens when what is compared is close enough the same that the difference can seem pedantic to many people.