Your best bet is to have some DME on hand when brewing all-grain, especially with a barleywine like that. You want to make sure that you have the correct OG to ensure that your hop utilization is correct for the recipe. If you undershoot the pre-boil gravity, simply add some DME in order to bring it up to the desired gravity.
You can definately boil longer, however, you're going to end up with less of a finished product. Adding DME is definately the way to go....
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