Occasionally I order a new culture from SDI, and I always get a packet of
dried culture measuring around a third of a cup. The instructions tell me to
mix all of this with flour and water, in a quart jar.

Wouldn't I get the same result if I just used a teaspoon of dry culture?

Then I could save the rest for future use. Mainly I just don't want quart
jars clutteirng my refrigerator.