Fetch current billing & Install fact bill period

Hello,

How do I get the below info:

1) For a given Contract Account or Installation as input, How do I calculate the current billing based on the valid to date of the billing record?

Do I need to pull it from ERCH table?

2) Based on above can anyone tell me what is meant by select all records from Installation fact where valid from date <= First day of current bill period.

I know that we can get Installation facts from table ETTIFN.

Please suggest.

Thanks Ricky