I'm getting the feeling I've been very lucky. :)

When being on an exploration trip around the trader location, I ventured a bit too much to the north and ran into the swamp of which I knew it was there (playing at the "boss seed map")

Decided to hugging the border between the BF and swamp and didn't take long before I saw not one, but two yellow plants which turned out to be turnip. So basically I started out with 6 seeds and I think I'm growing 18 turnips right now.

As for the math: I'm always using 1 in 3 veggies for seeds and the other two as ingredients for food. That way I'm certain I've got the same harvest at all times; while also having enough items left for food. Although with Carrots that's still tricky to achieve.

Thorin :)