i have successfully implemented a programme to convert plaintext to cipher text for the [amsco cipher] (<http://www.thonky.com/kryptos/amsco-cipher/>) . However i am not able to get how to go about for converting the cipher text to plain text. How do we determine the length of columns (or number of rows for each column) which converting the cipher to plain text ?
E.g if the cipher text is *EMAAE HUMBA LMNRE AUDSR RTSUN WHAVP TOEMH KITVE DGEUS TEATO SHOSO YHNME EKLAI* and the column key is 35142 then under column '1' the start letter is E and next row in the same column would be MA. The third row in column number '1' will have 'A' and so on. However how do we determine the total number of rows for each column ?

Thanks.