How does Alba's "list only" strategy work in Scottish Elections?

Question

The Wikipedia section on the Election method for the 2021 Scottish Parliment describes the set up as;

The Scottish Parliament uses an Additional Members System, designed to produce approximate proportional representation for each region. There are 8 regions, each sub-divided into smaller constituencies. There are a total of 73 constituencies. Each constituency elects one MSP by the plurality (first past the post) system of election. Each region elects 7 additional MSPs using an additional member system. A modified D'Hondt method, using the constituency results, is used to elect these additional MSPs.

This suggest a single vote is cast and the FPTP vote counts are used to measure proportionality for the list vote. Alternatively, several places including this BBC article state Alba will be running a list-only strategy, and the SNP will continue to run a pragmatic both SNP campaign.

[Alba Party Leader Alex] Salmond believes this means most SNP list votes are "wasted", and could instead be given to his new list-only party - which would have zero constituency MSPs, and thus a better chance of winning regional seats.

They are unlikely to win every single constituency, and thus must try to balance out any local defeats with list seats - from a pragmatic standpoint, their campaign has to be "both votes SNP".

This seem contradictory, either the list seats come from a second vote (as in the German set up) or they are based on the constituency vote. Which is it?

Answer 1

Voters for the Scottish Parliament get two votes:

• A constituency vote, to elect their constituency MSP
• A regional vote, to elect seven regional list MSPs

The regional votes are used to determine the total number of seats that a party wins in a region, including both the constituency seats and the regional seats. Mr. Salmond says it is these regional votes that are getting "wasted", because the SNP gets very few regional list seats.

For example, in Glasgow in 2016, the SNP won 45% of the regional vote; if the 16 seats were allocated purely proportionally, you would expect them to win 9 seats in total. Because they won in 9 constituencies, however, they would not win any of the list seats, leaving them to go to other parties.

On the other hand, if the SNP won 0% of the regional vote and all 9 constituencies, then the seven regional seats would be allocated proportionally between the other parties. This would give Alba 4 seats that the SNP would not have been able to win.

I did the sums in a piece I wrote last weekend, and in the best case scenario, it could give another 25 seats to pro-independence parties.