The first step is to setup the report so that it returns all records starting from the 1st of April up until the DateTo. You'd need to modify the selection formula and it should look something like this:
{Database_date_field} IN [CDate(Year({?DateFromPrompt}) - If(Month({?DateFromPrompt}) < 4 Then 1 else 0), 04, 01) TO {?DateToPrompt}]
You should now be able to insert a summary on a measure field and that should return the total for the current Fiscal YTD.
For other calculations that rely on the Date Range selected, you'd need formulas that look like this:
If {Database_date_field} IN {?DateFromPrompt} TO {?DateToPrompt} then {database_field}
Insert a summary on this formula field and it should return figures for this range alone.
-Abhilash