I prefer unsalted butter and will use it even if the recipe doesn't specify salted or unsalted. I feel most recipes have more than enough salt and the difference to use unsalted instead of salted isn't noticeable. I have used salted when it calls for unsalted and left out the salt. I think there is roughly 1/2 t. salt to salted butter so if you use a recipe calling for salted and salt added, you can add up to 1/2 t. salt to the recipe to adjust for your unsalted butter. Unsalted is preferred because the salt content of the butter varies from company to company and salt allows the butter to remain on the shelf longer and not taste old whereas unsalted will allow you to add salt to taste and generally be fresher.
Not only can you control the amount of salt you add in, but I personally have found that unsalted butter yields a RICHER, FRESHER, more buttery taste. Coincidently, UNSALTED butter has a shorter shelf life. I made several batches of classic shortbread for the holidays last year. For the latter batches I had to resort to using salted sweet cream butter because I had used up all of my unsalted sweet cream butter. The taste and quality of texture were slight, but noticeable. I won't bake with salted butter unless I have to. Ever try baking with Plugra?! It's a European style butter with a higher fat ratio than say Land O' Lakes. It yields decadent results, but it's definitely not a butter that you want to spread onto your toast every morning
I would say you can replace salted butter with 1:1 ratio. Like you can use 50% of the salted butter and 50% of plain butter. There are recipes i.e. (yellow cake) where butter creates the real taste. So you need to focus on butter mainly.