It's not about the size of your arse, it's about the width of your sit bones. Fabric has a thing on their site which gives you a saddle size range based on somatotypes, i'm not sure how accurate it is but it's a lot better than trying to measure the sit bones themselves which i've never had much luck with. Go to an actual store if you want that.
But yeah, padding will do nothing but exacerbate your issues. You don't want the saddle to "absorb" the pressure because then it will just go to all the wrong places, you want the saddle to be shaped so that the majority of the pressure goes through your sit bones which can actually support it for extended periods of time without limiting blood flow or even leading to injury.
Another key point - are you sure you've got your saddle height set correctly? Even a badly fitting saddle shouldn't be too bad over just 12 miles.