With 8 pounds of extract, the OG should be in the area of 1.070 if it was dry extract, or 1.057 if it was liquid extract. 1.100 is an incorrect reading- I bet the extract was not mixed up completely. That doesn't matter, as the yeast know where to find the sugar!
Ideally, you'd have used more yeast. But it'll be fine- the yeast will have to work a bit harder to reproduce before getting fermentation underway. Keep it in the area of 65-68 degrees if possible, and it'll get going within the next 2-3 days. Don't worry!
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