You are on the right track. I assume the 'per district' part of your question applies to using the District column as e.g. a row label in a pivot table. The number of accounts can be calculated with

NumberAccounts:=COUNTA(Table[AccountName])

The number of wins is the same, but filtered on Win = Y:

NumberWins:=CALCULATE([NumberAccounts],Table[Win]="Y")

and

Wins%:=DIVIDE([NumberWins],[NumberAccounts])