How much to reduce salt when using salted butter in place of unsalted 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