You have published some interesting figures, but without any indication of how they have been calculated.
I have a horrible suspicion that the whole cost of the Yell Sound ferry has been allocated to the “benefit of Yell folk”, the cost of the Bluemull ferry allocated to the “benefit of Unst folk”, etc. This would be clearly facile and fatuous.
For example, the Yell Sound ferry is a vital highway for millions of pounds of fish and seafood per week. The Yell folk only benefit from a tiny percentage of that figure.
Also, in terms of tourism, the Yell Sound ferry is simply a route for tourists to get to Unst, so they can tick off Muckle Flugga from their bucket list.
Most of them do not even stop in Yell, in either direction, and Yell Tourist Group is doing its utmost to find ways to encourage tourists to stop.
Very few tourists actually spend money in either Yell or Unst. They spend their money in Mainland Shetland.
There is a valid debate to be had about galloping centralisation in Shetland, but these figures, I suspect, add nothing to that debate and are frankly misleading.