Formula: To Buy or Not to buy a stock

Many people give me a formula on whether to buy a stock or not. I think the formula is too simple and we may miss out on many tips/ recommendation.

We all use "estimate" and "average" in the formula.

The common formula is:
(Probability of Gain X Percentage of Gain) + (Probability of Loss X Percentage of Loss)

Example 3 months:
Chances of the tips is real is 70%, and will make 20% gain.
Chances of the tips is false 30% (meaning 100% - 70%), and the loss estimated 50%

(70% X 20%) + (30% X -50%) =  negative 1%.

Because it is negative (or lower than your required return), you should not buy.
If it is above your required return %, then you can consider.

But the formula is too simple.

What if in medium term or long term, the stock can recover and make 40% in two years, why not?

Conclusion:
Do not buy if the result shows % less than your required return and the stock also cannot recover in medium term.

May consider to buy if:
1)Got tips/recommendation
2)If anything goes wrong the stock still can recover and make good % gain in medium term.
3)You are willing and able to hold for medium term


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Thanks