Operations with custom fields values in a loop

Here is a simple way to do this:

// First declare total and count before the loop
$total = 0;
$count = 0;

foreach($posts as $post)
{
     if(get_field('prod_fi')){ // If we have a value add it to the total and count it
        $total += get_field('prod_fi');
        $count++;
     }
}

echo 'Count: '. $count;
echo 'Total Sum: '. $total;
echo 'Average: '.($total / $count); // To get the average

EDIT:

In your case, where you have the $prod_query loop, hope this makes more sense 🙂

// Declare total and count before loop
$factorTotal = 0;
$factorCount = 0;

while ( $prod_query->have_posts() ) : $prod_query->the_post(); 
    $factorI = get_field('prod_fi'); 
    if($factorI){ // Only if it exists
        $factorTotal += $factorI; // Add it
        $factorCount++; // Count it
    } // end if
endwhile;

echo 'Count: '. $factorCount;
echo 'Total Sum: '. $factorTotal;
echo 'Average: '.($factorTotal / $factorCount);