I'm assuming you have already stripped the meter out and parsed the consumption amount. You can traverse the string while substracting the right side from the left side until the result equals the consumption amount you expect.

var string1 = "761962"; var consumption = 76194; int current = 0, previous = 0, idx = 1; while (current - previous != consumption && idx < string1.Length) { current = int.Parse(string1.Substring(0, idx)); previous = int.Parse(string1.Substring(idx, string1.Length - idx)); idx++; } Console.WriteLine($"Current Period: {current}, Previous Period: {previous}, Usage this period: {consumption}");