A bad way to analyse your promotions is by guesswork and instincts in business.
What you need is proper KPIs with metrics to compare the sales for each promotion. For example, do a $20 item with $2 off do better than 10% off?
The only way to find out is what we call A/B analysis. You do option(a) for a week, then option (b) for the next week.
Then here is a quick way of finding this out.
Go to the stock system.
Select Sales Promotion here
Fill in the dates, and now you can investigate.