I have no knowledge of retrodirect, but I do run a dual single speed bike. I have 2 different setups. I have a White Industries DOS ENO freewheel with a 16/18t combo and a 16/19t combo. I almost never use my 16/18 since my 16/19 gives me a much better combo. I run a 45/42t front road crank which gives me a nice crusing gear and a decent, but not that low climbing gear.

As long as you reduce/increase the number of teeth on your chainring by the same number as you reduce/increase the number of teeth on your freewheel/freehub your rear wheel will stay in the same position. The best way if you have real hills in your area is to run a freehub body and so you have endless options of cogs to run, as well as being able to dial in your chainline perfectly. If you want a cleaner look with a rear wheel with next to no dish, go with a DOS ENO/freewheel hub setup.

Pics of bike/gears/chainline are here:
http://bikeforums.net/showthread.php...light=langster

HTH