chmod for human beings!
If UNIX octal permissions get you down, this is the tool for you. Humans were never meant to do bit arithmetic in their heads.
npm i -g remod-cli
You can view and interactively edit the permissions of a file with
$ remod foo.txt
If you think that you want to change a file's permissions to 640 but you'd like to preview the changes first, just run
$ remod 640 foo.txt