Just read Making WordPress Stable on EC2-Micro

In the "Tuning Apache" section, I can't quite figure out how he comes up with his numbers for his prefork config.

He explains how to get the numbers for an average process, which I get. But then:

Or roughly 53MB per process...In this case, ten threads should be safe. This means that if we receive more than ten simultaneous requests, the other requests will be queued until a worker thread is available. In order to maximize performance, we will also configure the system to have this number of threads available all of the time.

From 53MB per process, with 613MB of RAM, he somehow gets this config, which I don't get:

<IfModule prefork.c>
 StartServers       10
 MinSpareServers    10
 MaxSpareServers   10
 MaxClients       10
 MaxRequestsPerChild  4000
</IfModule>

How exactly does he get this from 53MB per process, with 613MB limit?

Bonus question

From the below, on a small instance (1.7 GB memory), what would good settings be?

bitnami@ip-10-203-39-166:~$ ps xav |grep httpd
 1411 ?        Ss     0:00      2     0 114928 15436  0.8 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1415 ?        S      0:06     10     0 125860 55900  3.1 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1426 ?        S      0:08     19     0 127000 62996  3.5 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1446 ?        S      0:05     48     0 131932 72792  4.1 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1513 ?        S      0:05      7     0 125672 54840  3.1 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1516 ?        S      0:02      2     0 125228 48680  2.7 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1517 ?        S      0:06      2     0 127004 55796  3.1 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1518 ?        S      0:03      1     0 127196 54208  3.0 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf
 1531 ?        R      0:04      0     0 127500 54236  3.0 /opt/bitnami/apache2/bin/httpd -f /opt/bitnami/apache2/conf/httpd.conf