How can I specify a more appropriate font substitution?

Create a rule in your .fonts.conf file. If it's not in your home folder, create it. Here's a full example of a .fonts.conf file:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>

  <match target="pattern">
    <test name="family" qual="any" >
      <string>TimesNRMT</string>
    </test>
    <edit name="family" mode="assign" binding="same">
      <string>Times New Roman</string>
    </edit>
  </match>

</fontconfig>

If you already have the file, just paste in the part enclosed by the match tags.

If you're using Font Manager the file is in a different place(~/.config/font-manager/local.conf).

Maybe you’ll need to install MS Core Fonts.

sudo apt-get install msttcorefonts