I have a data set which has weekly spends of four types of marketing campaigns over the course of three years. I did a few quick EDAs and came to the conclusion that a multiplicative model would be appropriate. So I log transformed the variables and ran a linear regression. Now I want to calculate the ROI for each marketing campaign for all three years. So I went ahead and calculated the total spends for each marketing campaign for all the three years. I kept the spends of other marketing campaigns as zero(to calculate the sales coming from one particular campaign) and predicted the sales after which I calculated the ROI. But now I realize the model has been built to predict when the spend values are in the range of a few thousands and the value I used to predict the sales for three years is much much larger than that(around a million and half) and doing so is actually incorrect. So the question is I have built a multiplicative model at a week level and now I want to calculate the sales coming from a particular campaign for three years. How do I go around to do this?