I think you'll have to figure this out by trial and error because the commute is so short. I find that usually it's not the temperature that gets me, but the wind. If you're wearing a windbreaker and you're still getting cold, you need another layer (or a thicker/heavier windbreaker).
How I approach this: My ride is about 10 miles each way, but a good rule of thumb for me is that if I'm standing outside and am getting uncomfortably cool/cold (before riding) the way that I'm dressed, I'll probably warm up enough after about 10 minutes to make it "perfect." That being said, I have still been riding in shorts t-shirt in the low 50s in Denver which have been a bit cool, so I'll probably go to a long sleeve shirt or throw the sleeves on the bike vest soon.
My setup:
Summer: shorts/tshirt & bike vest
Fall: Shorts/longsleeve shirt & bike vest or bike vest with sleeves
Winter: Leggings, shorts, longsleeve shirt (or warm base layer if really cool), bike vest with sleeves
0-10 degrees: leggings, shorts, warm base layer, puffy layer from ski jacket, bike vest with sleeves (windproof). I also pack an extra layer of warmth in case I get stranded for more than a few minutes at a light.
I find the winter -> spring transition I'm in shorts sooner because I'm used to the cold weather.
Try the ski jacket next time you think it's too cold for the windbreaker. A 10 minute bike ride shouldn't get you too sweaty even if it's too warm in the jacket.