I agree, it's very dangerous to set a fixed amount when it comes to gambling. Gambling like you said is for entertainment and not an investment whereby you set a target to get a particular amount that you must make. Most people that do this are driven by this targets that they set for themselves and at the end of the day they end up losing because of their high expectations that they placed on it
It is called a "Challenge" in sports gambling if you're targeting a fixed odd with a fixed amount to achieve a certain profit margin. Is it not dangerous as the chances of winning with smaller odds are always higher.