You cant use just spacers really, you will also need longer bolts, as the original ones are long enough for your wheels. Not your wheels plus 12mm or so of ally.
Ive not spaced my Audi, but think the bolts are pretty short from memory. Last car i spaced was my 205, which i swapped from bolts to studs and nuts. As i despise bolts. Still unsure why manufacturers use them, theres a reason, i just dont know it.
Also you want hubcentric spacers really. These center the wheel properly. If you dont get them, its possible to get the wheel slightly off. My 205 rears needed perfect placement (non hubcentric, i lathed it off) or you could notice at low speed the back of the car 'bumping' up and down.