SEO forecasts are typically made using standard or customised click-thru-rate models built on your keyword ranking data and search volume data for every keyword.
You will need:
1. Up-to-date keyword ranking data for the keywords and pages that interest you.
2. Search volumes for these keywords.
3. A standard CTR% model to apply to the search volumes for each keyword based on how well you are ranking for that keyword. You can of course customise this for brand/non-brand keywords or at a keyword or keyword tag level.
4. Business metrics from Google Analytics or elsewhere, that show you how many and the value of the sales or leads generated from organic search.
5. A model (typically in Excel) that estimates your current organic visibility and allows you to produce estimates based on you making assumptions and estimates of how much you can improve rankings by and by when.
6. You can do this manually by gathering data from a keyword ranking tool like Authoritas, Google AdWords (or other suppliers of Search Volume data, Google Analytics (or another analytics provider) and Google Search Console.
7. Alternatively, you can automate this process by choosing an SEO platform with in-built sophisticated SEO forecasting and ROI calculators.