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If I know my CDA approximately. How would I then work out distance gains?
Plug the CdA and Crr numbers into a formula which predicts speed, verifying against the old number and known speed/power curve. That gets you a new speed/power curve, and assuming no change in power you can just work backwards from speed to time or distance. If you're really obsessive, you'll model the correct distribution of speed over the course allowing for gradient too.
You need to know what your CdA is already to calculate how much time it would save, or to give an educated guess. I'd save more time in a race from a 5w saving than you would, even if were previously producing equal times.