I'm to do a utilitarian calculation but nobody mentions where I find the values that I assign each "feeling" or action? Where can I find these values? If I don't find them, do I just make them up myself?
As per my comment above, a bit improved:
In e.g. economics utility may be measured, but is not comparable between individuals. That is because it is representable only in an ordinal scale (a BIG problem for economics, actually). That means you can express 'x is better than y', but not 'x is by the factor of k better than y'. Ethical values are the same.
The order of values often defines destinct societies and sometimes is forcefully shared within societies, though. One example of the latter is the german constitution where the values are ranked through the number of the article in the first 19 ones (human rights). In general, juriciary is the most reliable source regarding that.
But it's not like you will ever find an utilitarian saying "Oh well life preserving 10 points, preserving from torture...err...6,8734 and helping grannies like 3,2!".
I always assumed that those numbers were to be approximated as well as possible -- short of coming up with a machine for measuring Intensity of pleasure and pain, how else could we know? I think this is evidenced by the fact that the equations typically used for a hedonistic calculus are a. always symbolic and b. never quite right. For example, I tend to see that intensity should be multiplied by duration, when anybody who understands basic calculus understands that it must, rather, be integrated over time.
I don't think the development of the calculus serves as anything more than a thought experiment -- "here's how you should think about ethics. Think of ethics as an equation, a mathematical function that spits out a 'do X' or a 'do not do X.'" The details of the equation are only there to show you that such an equation can take many details into account, and appear quite valid on its face -- it does not seem that it is for actual use.