I never get in this act of calculating how much bonus should i get or how the bonus s calculated as it will only make me confuse. I think most of us really don't know the exact formula for the bonus calculation.
I just love to play with this free money and it is indeed good from stake that they give us bonus periodically.
That is the point, you shouldn't really "know" how you get it. The point is, you should be able to just focus on how you could make money by gambling, or at least have fun if you lose money. Those are the two options and two goals, anything extra is optional. If you are getting anything more from it, then you are doing something cool but not needed and not a must.
I personally care about gambling because it is a way of spending time, like watching a movie, or playing a computer game, or reading a book. I mean this one could make you lose a ton of money if you are not careful, but you could at least make something with it and that is quite important.