I'm in the process of building a quantitative trading model, I want to improve on the way in which I decide upon a look back length for the indicators. I understand the different pros/cons for very short and very long look backs, but rather I want to assess what the optimal length is between say 20-days, 30-days, or in between. I feel the choice of 20 or 30 is done without thinking to much and just because it is round, which seems lazy to me. Is there a better method?
I think you are having it backwards: Optimising your lookback period is a sure recipe for disaster because it introduces data snooping bias.
To develop a robust trading strategy you have to check whether it is sufficiently stable with different lookback periods (e.g. in a certain range). If results differ significantly that is a good sign that your system won't work out-of-sample!
I agree with you that many people do these things without thinking and are indeed lazy... this is one of the reasons why so many trading systems fail under real-world conditions.