This deserves an article, but for a new database, how do you know the rows per day? How do you know the distribution? Often you take a guess, but just as often that guess has no basis. You can guess wildly high and pad things, and what often happens is people start to then implement shrinks to keep the database closer to their guesses. Or you can guess low, in which case you need more space.

For an existing database, do you know the count of rows per day as an average? Almost no tables/schemas I see track this. They don't know. If I asked you right now, for your largest database, what is the rows/day could you easily get the info? Maybe on some tables, but not most of them.

My point is that SQL Server doesn't have a good method for tracking the rows/day. So your formula, while correct, isn't useful. Most people have no idea of distribution as well. How does the Employees table grow v the Tasks table, or the Time table?

I've done what Homebrew does for existing databases, track backup size. It doesnt' give you rows per day, but it gives you a rough average of the aggregate growth of data per day. If you track that, then you can guess in the future what you'll need xx months down the road.