I normally don't keep unsalted butter on hand since I use it so seldom. So when a recipe calls for salt and unsalted butter among its ingredients would it be alright to use salted butter and reduce the amount of salt? And if so, by how much should the salt be reduced? In other words, how much salt is typically contained in a tablespoon, ounce or gram of salted butter?
Depending on brand, it is approximately 1 1/4 tsp per pound (US), or a little more than 1/4 tsp per stick (4 oz).
For most applications, yes it is fine to substitute and adjust; you can just adjust the "salt to taste" step of your recipe in many cases.
There are a very few uses (such as yeast raised dough) where you want to be more precise. I would not use salted butter for a yeast dough by preference, but if I had to, I would calculate the amount of salt to remove from the other ingredients based on this ratio:
1 1/4 tsp salt / 16 oz butter
According to the folks at America's Test Kitchen, the salt content in butter varies and may increase or decrease the amount of water in the butter. If you use unsalted butter, there are few if any variations. I've baked muffins and cookies with both salted and unsalted butter and personally prefer unsalted butter in baking but salted butter when adding to a finished dish (vegetables) or making things like grilled cheese.