A few quick notes here on farming which is pretty straightforward, summarizing findings by therother and andrewt.

Factors involved for Farming Income (F):
1)Base farming (B) - ie 2.5% for Arretium (unique for each region)
2)Farming upgrade (U) - each upgrade confers a 0.5 benefit
3)Harvest (H) - poor = 0.95, ave = 1.00, good = 1.04, excellent = 1.08
4)Campaign difficulty (D) - easy = 1.125, medium = 1.000, hard = 0.920, very hard = 0.850

F = (160)(B+U)(H)(D)

For example, Arretium which starts with Land Clearance with a poor harvest on Very Hard settings will receive:
F = (160)(2.5 + 0.5)(0.95)(0.85) = 387.6 (in actual play rounded up to 388)