Pass a url parameter in your failed redirect for your server validations like: pagename?valid=false
Then for your initial value instead of setting it to: <?php echo(ValidatedField("page","field")); ?>
set it to something like:
<?php echo(isset($_GET['valid'])?ValidatedField("page","field"):"yourdefault"); ?>
This way if the page was just validated and failed it shows the validated value as you want, but if not, the url parameter is not there and it shows the default value instead.