Lsd; you do not want to get caught into too much detail too soon. You want to keep your preview superficial. You want to keep your preview short & sweet. Read Paul Scheele's comments on previewing & postviewing in the Photoreading book 3rd edition.
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ps; Paul Scheele says a beginning photoreader will take about 5 to 8 minutes to preview a book. A advanced photoreader will take about 1 to 3 minutes to determine whether they want to go further with the rest of the system.