Well, if you use the dingle and a double to give yourself two different gears, you might be able to keep the chainline acceptable. For example, if you're using the 17t/21t dingle, try using 50 and 46 tooth chainrings in front. This setup would keep the same number of teeth engaged at any given time, meaning you wouldn't need to use a different length chain or move the wheel in the droouts. Am I right about this? I think I am. This way, you could have a gear for climbing or for riding into headwinds (46x21) and a gear for descending or riding on long flats (50x17). Now that I look at those ratios, that spacing seems a little wide, but you get the idea.