Getting an offset up to 50 mm is not too difficult, but beyond that, it is not easy.
Interloc Wayback Seatpost - The solution to common fit problems, 50mm of setback allows for proper fit - Aluminum head with Tange Prestige CrMo shaft Item...
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I also find that on any bike I've ridden I need to push the saddle back as far as it will go or I get knee pain. I currently have 25 mm offset on mine, but wish I had more - probably a few inches more. I end up getting a very narrow saddle and sitting as far back on it as I can, or I put my feet further forwards on the pedals. The further back I can get, the more power I can put into the stroke and the more comfortable it feels. It also helps getting into a better aero position.