"I am using natural sentences"
Not natural sentences, but natural flow of the doc.
Think of it this way, if you were writing purely for people, if there was no such thing as a search engine, how many big bold headings, for readability sake would you have on the page with those keywords? I think that's what was meant by natural flow.
Don't think there's an exact science but I typically use my main phrase in meta title, h1 top of page, h3 subhead maybe like a tag line, couple paragraphs of text, then possibly one more H at the bottom of page, as in a call to action.
The way I do my SEO, most of my client pages get #1 - 5 ranking. Here's an
example.
"Electronics Affiliate Program" moderately competitive - over 1 million competing and I'm #3. So you can see how I do my H tags on that page. Again, not that it's the RIGHT way or only way, just an example.