I can keep 50% of my net salary a month (800$ for example), so I can just 800$*12 months, but this would be deadly wrong. Since probably I would keep it somewhere in the bank, with compound interest, also I'll keep something in gold, assets or stocks, or even crypto. Also, my boss will raise my salary by about 20% in a year, probably. How do I keep all these variables in mind while trying to calculate my income for years in advance? And what variables to keep? Is there any tools for that?
You do not know if the price of gold will go up or down, if the stock market will go up or down. You do not know if you will get a raise or you will get fired, if you will get a job offer at a better paid position. And that is without taking into account personal events, like winning the lottery, having to pay for a car repair, wanting to move to live with someone, having kids.
Instead, there are some rules of thumb that will help manage your economical situation better. Try to live within your means, do not get into debt unless it is an investment (not speculation), consider your options, keep money saved for emergencies / short term loss of income, avoid offers "too good to be true"...
That is specially true for the guy whose main income is his job and that does not have enough assets to rely on those to sustain himself (in other words, almost everybody).
Then you have to factor your personality and goals, because maybe sometimes it might be worth spending some money on you today than to live like a pauper to get some possible profits in the future. And your tolerance to risk and your economical information to manage investments.
What you can get with this is not a plan that will guarantee you an amount of X $/month in five years, but a good chance of making the most of what you have.