Here is the code I came up with, just in case anyone else finds it useful:
<?php
$tasks = get_sub_field('project_task'); $totaltasks = count($tasks);
$totaltaskpercentage = 0;
while ( have_rows('project_task') ) : the_row();
$totaltaskpercentage += intval( get_sub_field('project_task_percentage'));
endwhile;
$grouptaskpercentage = $totaltaskpercentage / $totaltasks;
echo round($grouptaskpercentage);
?>
