18. Know How to Track and Calculate ROI
To calculate your ROI, you need 5 things, the total number of impressions, the total cost of Facebook ads, the click rates or the response rate, Conversion Rate (you can get it from Google Analytics) and Average profit per sale.
What you need to find out is how much you are paying to get each sale.
Once you know this you can determine your profitability so you can decide if it makes sense to scale your campaign.