Code 39 Mod 43 Checksum

public class Code39 {

    /// Returns the expected checksum for the specified barcode
    public string ValidateMod43(string barcode) {
        int subtotal = 0;
        const string charSet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ-. $/+%";
        for (int i = 0; i < barcode.Length; i++) {
            subtotal += charSet.IndexOf(barcode.Substring(i, 1));
        }
        return charSet.Substring(subtotal%43, 1);
    }
}