After a lot of reading (mostly about euthanising quail) I came across a lot of evidence that suggested that a small dose of Co2 should be administered first to put them to sleep, then the concentration lifted to euthanise them once they are 'under'.
I have been using this technique with quail with some success......A very small dose and they fall over relatively fast and lay breathing as if asleep. Shortly I administer the full dose which then completes the task without any apparent distress.
I have noticed that if I give the initial dose a little too fast or too much you can see that it starts to distress them - From what i understand, Co2 in a concentration above 50-60% produces acid in their airways. A concentration of around 30% will put them to sleep without causing pain. The aim is to put them to sleep at around 30% before raising it to the level that causes them pain whilst they are conscious. I am not sure if this is actually correct.....but it appears to be consistent with my observations and happy to take on board any evidence that it is incorrect.
Not sure if this approach applies to rodents but it seems to work well for me and does not cause apparent distress. I agree with Pythoninfinite - I hate doing it, especially when you nurture them from eggs, watch them hatch, help the weak ones out of their eggs etc.....It is not easy hence why I make the effort to reduce the distress.
I use Soda Streams as my source of Co2 - There are so many of them sitting unused in relatives cupboards they are an easy item to source.