Tune up apache and php settings

I have Ubuntu 16.04 LTS with apache 2.4.37 (with mpm_prefork) and php provided with OS. There is also newest ISP done based on Perfect Server tutorial. I have 15GB of RAM and 4x 2,3GHz CPU. I would like to adjust settings:
a) from /etc/apache2/mods-enabled/mpm_prefork.conf

Code:

<IfModule mpm_prefork_module>
        ServerLimit             400
        StartServers                     5
        MinSpareServers           5
        MaxSpareServers          10
        MaxRequestWorkers         400
        MaxConnectionsPerChild   0
</IfModule>

and also from /etc/php/7.0/fpm/pool.d/www.conf:

Code:

pm = dynamic
pm.max_children = 5
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 3

I have used this script:

Code:

curl -sL https://raw.githubusercontent.com/richardforth/apache2buddy/master/apache2buddy.pl | perl

to determine values like RAM used by php-fpm and apache:

Code:

[ -- ] PHP-FPM Detected => Using 2390.74 MB of memory.
[ -- ] apache2 is currently using 1293.74 MB of memory.
[ -- ] The smallest apache process is using 10.92 MB of memory
[ -- ] The average apache process is using 19.52 MB of memory
[ -- ] The largest apache process is using 20.54 MB of memory

I have found somewhere on the Internet that systems which have more than 4GB of RAM should have:
StartServers 25
MinSpareServers 25

PS
There are also two another valuable commands to determine ram utilization by apache:

Code:

ps -C apache2 -o rss | sed '1d' | awk '{x += $1}END{print "Total Memory usage: " x}'
ps -C apache2 -o rss | sed '1d' | awk '{x += $1;y += 1}END{print "Total Memory usage (KB): " x "\nAverage memory usage (KB): "(x/y)}'