building resource fields
Crop Fields: Build all to levels 5,5,6,6,6,6, build grain mill level 1. All to level 6, grain mill level 2. All to level 7, grain mill level 3. All to level 8, grain mill level 4-5. One to level 10, Bakery to level 2. All to level 9, bakery level 3. All to level 10. The last 2 bakery upgrades are actually less efficient than building a level 10 residence, building 3 settlers, founding a new village, and building all of your fields up to level 8 (your crop fields actually make it back faster than bakery level 4 and 5 do.) The only reason to build further bakery upgrades is if you are short on CP and have all level 10 fields in all of your villages. I will not include the math here, because it would take up half the article.
Clay Pit: All to level 7. One to level 10, Brickyard to level 3. All to level 8, Brickyard to level 4. All to level 9, brickyard to level 5. All to level 10. This is the same for Woodcutters and Iron Mines.