Service tax advance

Hi All, I have a doubt in Service Tax and I wish to have your valuable opinions As per rule 4A of service tax rules, invoice shall be raised within 30 days from the date of completion of service or receipt of any payment towards the value of such taxable service provided or to be provided. So, it is certain that invoice is to be raised within 30 days from the date of receipt of advance money for the service to be provided in future. My question is when only 20% advance is received, do we have to raise invoice for entire taxable value or only to the extent of 20% value? The wordings used in the said rule is " for the services to be provided" so should this be interpreted as to raise invoice for full value of service upon receipt of 20 % of advance money? Thanks in advance!!