Without distracting from your great message, I do want to point out to others that "double every 7" is based on an assumption of 9% annualized compounded growth and does not count for the inflation effect on the actual value of that money when you need it.
It is a possibly misleading shortcut to the mathematical truth that an amount compounded will double every number of years determined by dividing 72 by the expected annual growth rate ("the rule of 72). The historical norm for stock growth is 7-9% but assuming you have some assets in bonds or other more stable funds, that reduces your number. So if you have 80-20 stocks to equity at the start, and move towards 60-40 as you age that alone take a bullish 9% estimate and pushes it to 7ish. And if you want to start with the conservative range of 7% that gets closer to 5.5% annualized return. Now you have to throw in inflation effects - if you say 2.5% (the historical average and Fed stated target) you are actually doubling the buying power of your current savings every 24 years (72/net 3%). That leads to a very different view of what is needed.
Of course, you may have a different view of inflation, of stock/bond mix, of expected market returns, etc. But on a real dollar basis (meaning accounting for inflation) almost no one will realize a doubling of assets every 7 years over a period of 20-30 years. And we haven't even talked about tax drag on yield. So, it is great so many people are starting to take charge of their finances at earlier ages and in doing so to fit their own wishes and values. I just caution folks about some of the oversimplified and overly rosy advice floating around these days.
If you have to keep it super simple I suggest a version of
@ElkFever2's advice. If you are conservative take your needed annual retirement income times 33, if you are middle of the road take your needed annual retirement income times 25, and if you are bullish take your needed annual income times 20. This one calculation will get you as close to determining your nut as any overly-simplified "black box" number can.