Coming Up With Your Own Growth Rates

How do you guys come up with your own growth rates when you forecast line items on all 3 statements? I know this is broad but say, how would you make an assumption that Revenue will grow by 3% for 2 years? How do you come up with that 3% and how do you come up with the time frame? Do you look at COGS? GDP? Prices? How would your growth be impacted by politics? **How do you quantify in percentage form the factor that impacts growth of a line item? ** Just trying to understand how to critically think and make my own assumptions. I took a Financial Modeling course but the instructor almost always copies a research equity reports growth drivers.