Totally understand, and I was doing a bit of hand waving in the original comment. 7% is around the inflation adjusted, long term S&P 500 return, meaning $310/month would need to be adjusted for inflation over that timeframe.

Converting 2020 dollars (because times are weird right now) to 1960, it comes to about $35.50/month. On $500/month income, that's a 7% savings rate.