FYI, I built this formula from the source code of an Ethereum calcultor: https://github.com/badmofo/ethereum-mining-calculator/blob/master/index.html

Extract from the source code:

    function EtherMiningCtrl($scope, $http, $log) {
      $scope.ethPrice = ethereumStats.priceUsd;
      $scope.netHashGH = (ethereumStats.difficulty / ethereumStats.blockTime) / 1e9;
      $scope.blockTime = ethereumStats.blockTime;
      $scope.earnings = {};
      $scope.computeProfits = function() {
        var userRatio = $scope.userHash * 1e6 / ($scope.netHashGH * 1e9);
        var blocksPerMin = 60.0 / $scope.blockTime;
        var ethPerMin = blocksPerMin * 5.0;
        $scope.earnings.min = userRatio * ethPerMin;
        $scope.earnings.hour = $scope.earnings.min * 60;
        $scope.earnings.day = $scope.earnings.hour * 24;
        $scope.earnings.week = $scope.earnings.day * 7;
        $scope.earnings.month = $scope.earnings.day * 30;
        $scope.earnings.year = $scope.earnings.day * 365;
      };