The Preview stage usually take about 8 minutes for the average 200 page book first couple of times. Just remember it is a process whereby you gain some familarity with the book and give yourself the opportunity of redefining your goals. Even deciding if you are really interested in the book. For the first time you'll expect to spend a bit more time on it. Remember many things get better with practice so expect to spend less time on it with each book. At this point you're still trying to get the feel for it.
The idea is just to scan for things that seem to pop out to you. 20 pages is an approximation. I just grab a few pages and give it a quick glance. Words in italics, underlined in bold in the middle of a paragraph, from subheads and headings are trigger words. Also any odd word that just grabs your interest. Trigger words no matter what you write down are all good. Let go of judging them. You only need one or two that will help you to later activate the book other important words may show up during activation so if you find 15 to 25 trigger words in 5 to 8 minutes good and if you only find 8 go with that. If they are words you found in the book then they are perfect trigger words.
Now we know that practice will speed up the process and first time we may fall short of the mark we set ourselves... no point in beating ourselves up right? 
On the whole you're heading in the right direction.
Just keep going with it play with it and see what happens.
Alex
