Change those arguments from 750.0,80% to 250.0,30% (for Warning), and you can leave 1500.0,90% for Critical. Also be sure you have enabled Warning notifications then.
You can run the check_ping plugin from terminal, like this to see all it's options:
I'll explain those arguments here for you:
command followed by a ! and a value means that those are arguments piped to the command. In this example it means check_ping command has two arguments:
$ARG1$ is 750.0,80%
$ARG2$ is 1500.0,90%
Those arguments are used in command definitions which you can see in /etc/nagios-plugins/config/.cfg* files (or anywhere your config files for plugins are located), in this case it would be ping.cfg
In it you should find command definition for check_ping which would state something like this:
# 'check_ping' command definition
command_line /usr/lib/nagios/plugins/check_ping -H '$HOSTADDRESS$' -w '$ARG1$' -c '$ARG2$'
Where -w means Warning threshold and -c Critical threshold.
All those options you can see when you run the command manuall from the terminal with the --help switch
Default number of sent packets is 5