I've taken a look at the spreadsheet and your conclusion looks OK, however a couple of comments if I may
Firstly as Kurz taps the Prius experience is very much something "different" to any other car. The location of the component that generates the "field of smugness" remains a mystery but it works especially well
Your assumption you will match EPA/over in the Prius is probably conservative - although Mrs A is not doing well (I've given up tracking it
) she is about 15-20% up on where she was with the Octavia on straight MPG, plus of course petrol is cheaper. Even on the motorway if you are not in a hurry (say in the 50-65 range) it will do very well, its only if you go over that (such as dashing to England and back) that it starts to dip. Even then we have never seen much under 40 which is better than the iS does on your sig.
Around town you will get much better figures and if you Hypermile as you do then 113% will be a bad day
I've adjusted your sheet so if you change the % on the right it adjusts the Prius figures - somewhere in the mid 130s% they get to be the same per mile
I nearly got one when I got George - why did I get George instead ? For me the deal was about saving money overrall - lower insurance as well as tax and not spending a lot of ready cash which is lucky now given our new budget
I would consider one now though.
Have a test at least.
And then buy a 320d compact