Stronger beer and priming sugar

I have a six and half gallon carboy. I brewed an English pale ale and wanted to see if I could make it a little stronger so I added priming sugar (Dextrose). It was about a half cup. Is this the reason that during fermentation it bubbled over so bad that I had to switch from an air lock to blow off tube. For about a half of a day it was really bad and now calmed way back down. Did i mess up really bad? I figure it will be fine but I just want to know what it foamed so much and if this was ok or not to use. I know the best would probably be molasses but i was experimenting.

I have done 4 brews so far and I have been happy with all.

Just got my kegerator up and running. Its great!!

1/2 cup Corn Sugar isn't going to make a noticeable difference. You're fine with the blow off as well. Your vigorous fermentation might be due to temperature. What was the temp of the wort when you pitched & what did it ferment at?

Thats not going to mess it up at all, I've done this before and like hamiltont said, 1/2 cup is not going to make much of a difference. I "boosted" and Irish red last year by adding an extra 2lb's of dark brown sugar dissolved in some water in mid ferment and yes the yeast went into overdrive, the resulting batch was so good I couldn't even let it go the whole 3 weeks to bottle condition.

1/2 cup is close to what you would add to the beer for bottling-when you use it for bottling you dont get an increase in alcohol-youre straight. i would be happy that you got such an aggressive fermentation!

A half cup is probably just over one gravity point.