i would do at least 30mg, most do more, realize it is a mild steroid, d-bol is much stronger and some people take 30mg of that and up, so see 20mg of anavar is real low.
The dosage depends on your experience with AS. I've done only 2 minor cycles so when I took 30-35mg of anavar a day my strength went up nicely. I only gained 1lb in 3 weeks. I don't eat much protein though.