Hello,
750 is the new default for the public_html in 1.21.3
If you want to give new users error pages, just throw them into the /home/reseller/domains/default/404.shtml (eg) files. They'll be copied for a new install. You can also copy them into /usr/local/directadmin/data/templates/default .. so they'll be copied for new Resellers into their default folder.
John