Well, I wasn't saying how it should be done, but how I do. So, don't come to tell me, what's wrong and what's not.

Though I know my strategy is a consequence of some kind of neurosis. I'm such perfectionist I can't handle to have a single black tile. I know it costs me more to prevent it, but I can't help it, I must have no devastation in my reign. This devastation is a fixation of mine from RTW already. I logged on because of this devastation neurosis, even though I was lurking around for months.

Income of farm upgrades are minimal though (50 fl for an upgrade), you do the count how many upgrades one needs just to support a simple unit of mail knights and how much each upgrade costs. But it's true that farm upgrades are important for population growth, and here I loose more because of my irrational neurosis.

To answer the question: as far as I know devastation is a procentage from overall farm income, so it can't get higher than farm income but it can get higher than an upgrade.

@Flavius
Btw, besides basic roads the next thing I build is a church/mosque. It helps you preventing the spreading of other religions and heresy, it gives you a point in population growth and a point of chivalry for a governer which translates to another point in population growth. That's how I do, not to say what's wrong or right.