First mistake was your DME ratio... it's 10:1 (10ml water to 1g DME).
There are 453.5 grams in a lb.
You used a half pound, so 227 grams.
That is enough to do 2.27 liter starter. You did a .9 liter starter... that's why it foamed over.
Second mistake was not checking the OG of the starter... it should be 1.040.
Third mistake was using a bubbler... like others have said, it's unnecessary, and not suggested. Use foil.
so my LHBS may have been a little short on directions