Pool utilization / log-threshold

[Leandro]

Hi guys , while reading the DHCP distribution documentation, Ifounded 
these statements:

log-threshold-high \percentage\
log-threshold-low \percentage\

I think it would be very useful to inform about subnet leases usage but 
could not make it work yet.
After adding for example:
log-threshold-high 90;
log-threshold-low 70;

Daemon refuses to work an complains "semi colon expected".
After add just both statements without values:
log-threshold-high;
log-threshold-low;
On my general options it worked.
Now .. I don't know what to expect in log files. I consume all the 
leases but could not find something related to a warning messages or 
something, so,
I would like to know:
How to properly declare with threshold values:
Where should thos lines be declared?, I would like to use same value for 
every subnets.
Where are those messages loged ? Im sending operational dhcp message to 
/var/log/dhcp  file and error messages to /var/log/messages file.
Can anyone provide a message example?
Im using isc-dhcpd-4.1.1-P1, is this feature supported?

Thanks !!
Leandro.