That's almost the way I decided on the mix currently used. I noticed 40:1 smoked and 50:1 didn't so that's where I settled.I added unmixed gas to the tank (unmeasured) until the smoke in the exhaust was barely there. I thought better of it and added more oil (to the tank and unmeasured) until a little more smoke came back.
I read a white paper once that stated 20:1 creates 150x more polution (more than an SUV) compared to 50:1 which also factored into my decision. I'm now planning on looking at synthetic oil at 100:1.