Common sense. There is no "good" keyword tool that exists that will give you accurate counts, so you kind of have to go on experience.
I use GK to get ideas, for example I would find out the synonyms for a medical technician and then I take all these keywords and spin them into different combos throughout my pages.
Eventually you will get a sense of what kind of keyword phrases will get hits, but I don't know of a mathematical formula.
Usually I just write the articles first adding more general keywords, then when I go over it the second time to proofread I will use latent semantic indexing.