In different SlackBuild scripts there are lines like

chown -R root.root .
find . -perm 777 -exec chmod 755 {} \;
find . -perm 664 -exec chmod 644 {} \;

(...)

chmod 755 $PKG/usr/bin/*
chown -R root:bin $PKG/usr/bin

Of course I know what these command do.
But when I'm building my own packages, for applications that
have no 'official' scripts, how do I know what permissions
to set for certain files?

Are there some rules of thumb?