Jon
Because any ad has a different CTR, the only way to do it is by slowly increasing your bid, sit back and wait. Repeat the process untill you obtain #1 position. Once you get the #1 spot, it will effect your CTR as well (it should go up). Therefor, slowly decrease your bids again untill you're #2 again. Now you have a rough idea on how much you have to bid to keep your #1 spot. (suggesting that, from then on, all the ads CTR's remain the same)
Hope this helps!
