I think you need to come at it from the angle of what keywords exist prominently in *your* pages text. Those are the target keywords for better or worse really.
Your question is basically what an SE would want to know.. "based on this pages text and links etc.. which keywords should this page rank #1 for?"
Not easy, lol
Instead, try starting with a keyphrase you wish to rank well for. Then build up relevant text and links to what may be considered sites who are an authority on the subject perhaps.
If you went to look up "history of blue widgets", you would expect a page to be ranked well with a detailed history of blue widgets, and links to more documents that support the claims of the page.