Query to Find Out How Many Days Until Database Is Full
You will need to start building a history of the dbsize vs current_usage metrics by day (or more incremental if needed).  Only then will you be able to make educated calculations for projected dates.  

Otherwise you'll essentially just be setting a "hard-coded" linear curve that won't really be precise enough.

Basically, have a stored procedure that takes a snapshot of the current dbsize, current_usage and inserts it into a historical table.  Once you have enough data, you'll be able to use that data to make your projections.