This worked for me:
string Connection = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source="
+ HttpContext.Current.Server.MapPath("\\myPath\\myFile.db")
+ ";Extended Properties=\"Excel 12.0 Xml;HDR=YES\"";
I'm querying an XLSX file so don't worry about any of the other stuff in the connection string but the Data Source.
So my answer is:
HttpContext.Current.Server.MapPath("\\myPath\\myFile.db")