I understand where you're coming from, and have created my own Excel spreadsheet for my needs.

Having said that, expecting your expenses to stay as they are (even adjusting for inflation) for multiple decades is a very aggressive assumption. You could have kids, they may want to go to a private university, you may have severe health issues, you may decide you'd like to travel more, etc.

Life tends to find ways to throw us curveballs all the time, so expecting everything to go your way is hyper risky IMO.

As for the 7% average return, it's based on the assumption that you follow the recommended allocation between stocks and bonds. If you invest 90-100% in stocks (e.g., via ETFs or mutual funds), you can expect better average returns, but you're risking a major loss at a critical juncture when you're close to retiring.

To avoid that risk, you'll need to start transitioning your allocations to more conservative ones as you approach your target date.