don't you read the potentional ABV scale before you pitch the yeast? I know I've used it that way and it was close to what beercalculas said my est. ABV would be. I personally don't care for ABV, but do know that my last IPA I used all my scrape malt so it came out 8%, but wasn't the best tasting and it felt like it had a heavier body from the extra malt.
The Potential ABV scale is designed for wine, which ferments out much more completely than beer does. If you read 8% before fermentation, it's unlikely that you actually had an 8% beer in the end.