I'm the one who just went and updated the flour page on the wiki. I did some testing with an object inspector to look at nutrition and such. Grinding flour from grains, seeds, or roots keeps the same nutrition values as the original, as well as the same weight. The carb, fat, and protein values given are percentages of the weight that is each nutrient (i.e., grams of nutrient per 100g weight).
Barley gives the most nutritious flour, at ~1370 calories per pound. Rye, clayweed, and lake reed flours are all above 1,000 calories per pound, while hemp (seed), marsh calla, and bogbean flours are all just below 1,000 calories per pound. You
can make flour from nettle, turnip, sorrel, and yarrow seeds, but it's much much less nutritious than the other kinds of flour.
If you want to figure out what to plant for the most calories, then you need to account for the number of plants that can grow in each tile and the total number of calories produced by each plant. Note too that you've got it backwards: rye produces three fistfuls of grains, and barley produces two, not the other way around. On a per-plant basis:
- Rye produces 412 calories per plant (~1/3 pound of grains)
- Broad beans produce 378 calories per plant (~3/4 pound of beans)
- Barley produces 302 calories per plant (~1/5 pound of grains)
- Hemp produces 218 calories per plant (~1/5 pound of seeds, ~1/7 pound of leaves)
- Clayweed produces 143 calories per plant (~1/7 pound of seeds, ~1/13 pound of leaves)
- Turnips produce 97 calories per plant (~2/3 pound root, negligible size seeds)
- Peas produce 57 calories per plant (~1/7 pound of peas)
- Nettle produces 23 calories per plant (~1/10 pound of leaves, negligible size seeds)
- Sorrel and yarrow produce negligible calories per plant
But for planting, this ordering gets shifted again because you can plant many more turnips and grains in a single tile than beans:
- Rye produces at most 10,288 calories per tile
- Barley 7,560
- Hemp 2,529
- Turnip 1,944
- Broad bean 1,892
- Clayweed 711
- Pea 284
- Nettle 227
- Sorrel and yarrow negligible
So: grow barley if you want the best flour (most nutritious breads, porridges, and stews). Grow rye if you want the most total calories of flour.