dovecot - Mar 2014 - pb with setting quota_rule=backend quota=fs for quota-warning

Hello

my config is debian wheezy with dovecot v.2.1.17-2., 
plugin quota /imap_quota is ok : I can read the real filesystem user quota for
each different user with
	- doveadmin quota get
	- thunderbird
	- SOGo     

my problem is to configure quota-warning message :

1) if no quota_rule is defined then my dovecot-debug.log looks like 
Mar 25 18:28:02 imap(testuser): Debug:Quota warning: bytes=0 (80%) messages=0 
...

2) As soon as I set  quota_rule = *:storage=1G    , quota root is correctly 
defined, and quota-warning messages are sent according to the ratio I defined
(cf below 90-quota.conf)
with THIS limit (1G) , the same for all users and that?s not what I expect when
I set individual quota...

3)And If define something like: 
	quota_rule = * : backend  (or backend=%q or ?)  dovecot  definitively hangs
with

a) doveadm quota get -u testuser
doveadm(testuser): Error: user testuser: Initialization failed: Failed to
initialize quota: Invalid quota root quota: Invalid rule *:backend: Unknown rule
limit name: backend
b) even my login in SOGo hangs /var/mail/mail.err
Mar 25 21:56:22 biwa dovecot: imap(testuser): Error: user testuser:
Initialization failed: Failed to initialize quota: Invalid quota root quota:
Invalid rule *:backend: Unknown rule limit name: backend

Is there a way ( a variable ?) to link the real filesystem quota limit with the
quota_warning process ? ?
something like quota_rule = *:storage=%VAR ?  

90-quota.conf    
-------------------
plugin {
  quota_warning = storage=80%% quota-warning 80 %u
  quota_warning2 = storage=90%% quota-warning 90 %u
}

plugin {
  quota =fs
}

service quota-warning {
  executable = script /etc/dovecot/myconfig/quota-warning.sh
  unix_listener quota-warning {
    mode =0666
  }
}

Thank you in advance
best regards, 
Philippe

______________
Philippe Marion