It's all relative. Thinner oil flow better than thicker and pumps more easily. If a 21mm setting hits the correct ratio w/ oil A. And oil A has -54F pour point, that same 21mm setting will have a different ratio as oil B. Which has mabey a -32F pour point.
When I adjusted mine, I ended up going out to 23.5 mm to achieve a correct ratio. I think it was probably somewhere @ 45:1 with that setting.
In short, once you have it set to where your comfortable, stick to the same oil to maintain that ratio. Because all oils have a different pour point, and pumps have an easier time w/ thinner oil than do thicker oil.