If your ROI is some how not expressed in dollars or using a metric which involves the number of likes or followers, then it’s probably not a valid statement on ROI.
No matter what the marketing math problem, from my experience, time is the most interesting/important variable. Trends, deadlines and results are typically time based and ultimately anything that qualifies as fact based is typically long on the time variable. So here’s some conceptual risk management modeling/math for fun and enjoyments – it’s more about concepts in math than the actual formula. Wish I learned that one earlier….