“How much fill would I need to feel warm at -20 (with only the light shirt underneath)?”
This depends largely on body chemistry and your activity level. I would look for a garment that would fit the average temperatures of my locale. According to Wikipedia that would be 30 *F in January. Occasional cold stretches at 14 *F and -4 *F. Wind chill can push the temperatures down to your suggested temperature rating of -20 *F. At that point for an ‘urban’ jacket I’d be supplementing the garment and avoiding exposure as much as I could. You wouldn’t be facing life or death conditions as someone would in the backcountry with an under-performing insulating garment. Likewise choosing a -20* garment will likely cause you to overheat during the majority of the winter which has it’s own concerns in keeping you warm.
When living in Fargo, North Dakota, layering was the key to keeping warm when the wind chill would send the temps to -40* or colder. At that point the whole system was important. Gloves, Hat, Long underwear, and Scarf/Face-guards were essential in keeping you warm much more than the jacket itself.
Wikipedia-Toronto