Sigma f1.8 autofocus issue on 650D

I'm having focus issues shooting wide open (without recomposing) and I'm trying to decide between returning the sigma and getting the USB dock to make focus adjustments (since my rebel can't).

I know this lens has a history of autofocus issues but I'm attempting to quantify them.

I setup a Tim Jackson chart at home with the following conditions:

• f1.8
• 35mm focal length
• iso 400
• 1/60sec shutter
• tripod mount
• 45degree angle
• 2ft distance to target

I was getting consistent front-focusing (mild, but enough to be a problem). I confirmed with staggered cereal boxes parallel to the focal plane as I read there is disagreement about valid test methods here.

I brought the same setup into my office because I wanted to demonstrate for a curious co-worker. All of a sudden it was nailing it's target consistently. The only difference was slightly better lighting:

• iso 200
• 1/125sec shutter

Also the lighting was halogen at home versus fluorescent at the office.

Not sure what to do next.

1) A lot of the times when I want to use f1.8 it's during lower light, so does it make sense to tune the autofocus for low light with the dock and give up accurate focus in better lighting? Or maybe calibrate to a midpoint as a compromise?

2) Return the darn thing because it's a lemon (not sure if other copies of this lens, or other models of f1.8 lenses perform any more consistently with the 650D... is this a low light PD sensor problem inherent in the body??)

3) Do more/better tests? I know I should be looking at multiple distances and focal lengths, but aside from that...?

4) Admit that I'm being OCD and just accept this lens as it is?

5) ?