“Whatever your age, whatever your income, how much should you be worth today?” is the question asked by Thomas J Stanley & William D Danko in ‘The Millionaire Next Door’, a book that retraces the success stories of self-made American millionaires. After years of surveying high-income and high-wealth individuals, they have developed several formulas for calculating wealth. One of them is for determining whether your wealth level is where it should be today. 
The formula is simple: multiply the age by the realized pre-tax income from all sources (excluding inheritance) and divide by 10.
For instance, if a person is 41 years old, earns $143,000 a year and lives in the United States, he will be considered wealthy if the net value of his current assets exceeds 
$586,300 [(41 X 143,000) / 10 = 586,300]. If a person living in the U.S. is 61 years old and has a total realized annual income of $235,000, the net wealth should be $1,433,500.

Although there are no absolute promises, this rule estimates the expected annual returns for equities, bonds, and cash over the long term. On average, they should be 10%, 5% and 3% respectively (reference currency: US dollar). 
However, this rule is mainly valid for a time horizon of around 15 to 20 years. Furthermore, the 10-5-3 rule does not take into account market volatility factors.

As the very name suggests it, it is a wise practice to keep between six months and one year’s worth of expenses in an “emergency fund.” The expenses considered should include the cost of accommodation and utility expenses  (food, transit, insurance, etc.). This amount may seem overwhelming at first, but the point is to set aside a small amount each month to achieve this goal and adjusting it according to obligations related to family needs, job stability and other considerations.

It is best to place the "emergency funds" in an interest-bearing account, such as a money market instrument or savings account, from which liquidity can be easily accessed without incurring penalties or taxes. Segregating this emergency "nest egg" in a dedicated account avoids forced sales of risky assets when you need to withdraw cash.

The "100 minus age" rule is an approach for determining the strategic allocation of your investment portfolio. In other words, it involves determining how much of the portfolio should be invested in equities and how much in bonds. The proportion to be invested in equities is the result of deducting the age from 100, with the remaining amount invested in bonds.

For example, if a 25-year-old wishes to invest $1,000 per month, according to the 100 minus age rule, the percentage to be allocated to equities would be 100 - 25 = 75%.  Thus, $750 should be invested in equities and $250 in bonds. Similarly, if a 60-year-old wishes to invest $10,000 per month, the fraction allocated to equities would be 100 - 60 = 40%. This would mean investing $4,000 in equities and $6,000 in bonds.

This rule reflects the logic of a higher allocation to equities when the saver is at the beginning of his working life, and a lower allocation when the individual is approaching retirement. Indeed, in the event of a sharp fall in the equity markets, a young investor will have the opportunity to recover his losses in the next income stream and/or in future bull markets. In contrast, an investor close to retirement may suffer a stock market crash at the worst time of his life, i.e., when he needs part of his assets to compensate for the imminent disappearance of his work-related income. 

Most people entering working life consider savings to be their last priority. And yet, building up savings early in one's career can have a major impact on the capital available at retirement. 

According to experts, the optimum is to save 10% of your current salary. Let's take the following example: a person starts working at the age of 25, earns $5’000 monthly and saves 10% of his salary every month over a 20-year period. Assuming that his salary grows by an average of 10% a year, and that the fund in which the savings are invested performs at an annualized rate of 8%, the capital available at age 45 will be $642,379.
In addition, it is recommended to increase the savings percentage each year, taking into account additional expenses and the inflation rate.