Betterment's calculator is post tax AND post inflation. Their portfolio is also internationally diversified, which also lowers returns. They assume about a 6.8-7% rate of return before inflation, which means it is something like 5% after inflation.
This article shows their assumptions, although I do believe it is PRE inflation. . Dave Ramsey assuming 9% is not bad but also a bit misleading as that is pre inflation.
I don't consider either of these to be bad. Some of the really high 9%,10%, and even 12% future expected returns are based on the recent run of US large cap value stocks. It is irrational to assume that this will continue in the future even more so if you are as underdiversified as to only invest in the SP500. Even investing in VTI is underdiversified as the market cap weighting underweights small cap stocks.
Betterment overweights to value, over weights to smaller cap, and internationally diversified you. IT is probably one of the better weightings for maximized risk adjusted returns, highest diversification, and greatest assurance of outcome. Other mixes lose one of those things.
Ok so using portfolio visualizer, I put in 8k and added in 275$ monthly, with no inflation increases. After 40 years I got 614, after inflation, and, after 45 years, 859. There were huge variances however. The lowest value, at 45 was 451k and the highest was 1.5 million. The real expected returns were 6.8%. And this was based on real data with betterment's actual all stock portfolio. Bond percentages would mean lower returns but less variance.
I also use the Betterment retirement projector, it is one of my biggest gripes. That being said. I like the platform as a whole and consider it the best investing solution, even over vanguard. As the decisions it encourages will help you be a better investor. I also am a big fan of Dave Ramsey, as his personal finance is great and got me to 78k right now (95k including my EF and car savings).
If I wer you I would try to increase your retirement contributions just a little bit. If you can get to 500$ a month by age 30, you will definately retire a millionaire. 1k a month even better. Also bear in mind that, in the future, your income will go up as you increase. Calculators can not account for that.