function lettersToDecimal(letters, aChar) {
        var aCode = aChar.charCodeAt(0);
        var decimal = 0;
        for (var i = 0; i < letters.length; i++) {
            decimal += (letters.charCodeAt(letters.length - i - 1) - aCode + 1) * Math.pow(26, i);
        }
        return decimal;
    }
converts spreadsheet style letters to a number
@param {string} letters - the letters
@param {string} aChar - the character representing the start of the sequence, e.g. 'A'
@returns {number} - the numeric value