Well, if you want to increase it till 500 instead of 318 then do the following query in your UNA database (backup this table first):

UPDATE `sys_transcoder_filters` SET `filter_params`='a:2:{s:1:"h";s:3:"500";s:10:"force_type";s:3:"mp4";} ' WHERE `transcoder_object`='bx_timeline_videos_mp4';

And then clear all caches via Dashboard.