There is a great resource on the Geforce dot com page about setting up your NVIDIA graphics card for Fallout 4. It walks through all of the various graphics settings and its affect on frame rates when you change it. i Was able to turn down some of my settings without any loss of resolution to speak of.

Just click on the games tab, find Fallout 4 in the list, then on the left click on guides.

You could also use geforce experience, just saying

i have that as well and that is definitely a nice tool. This page actually shows you examples of change, like 16x vs 12x.. etc and graphs that show that differences in frame rate for each setting. Its pretty geeky i know but i found it useful.