Second home tax - min rent declaration during vacant period

Dear Experts,

I have a second home in Bangalore (First home also in the same city which is the Primary home).I am claiming the second home tax benefit.My house is vacant from April to September.From september,2013 the house is on rent till date.I came to understand for the second home even if its vacant i need to declare a minimum rent value as per some guideline.I need help in deciding that min rent value for the vacant period for the year 2013-2014.In this case How i can i conclude the rent value for that period.

Thanks in advance !

Thanks
Suvv