I usually do that for planets with many tiles and a 100% or 300% Farm bonus. I don't build too many Stock Markets though. It's just more effective if you have a high population to get taxes from. On my current one, I have an Economy Capital on there and the Evil building that raises it by 100%, with a few Stock Markets to round it out. Population cap is 27 B, but it takes a while to get there since I use it for my transports. I have another that's maxed out at 17 B. 100% morale for both.

but what every one in this tread is missing is the factories, labs, and super projects that realy make a differance.

I might not get the most money from my ppl, but i don't tax them to death, and once i am past the sarting techs, i resarch up a varity of techs that help in an undecided game. i.e., if a tech war starts up with my race, and another, then my uped reaserch ability is already to handel that.

but what every one in this tread is missing is the factories, labs, and super projects that realy make a differance.

This thread is about the tradeoff between stock markets and farms for maximizing your income on a dedicated tax world. We're not talking about factories and labs because that's not the point of the thread. I'd have thought that the first post might have been a subtle clue about that?

Anyway, income bonuses are additive, not multiplicative. The square root of your population is what determines your base income. What that means is that the first farm is especially beneficial for your income, since it more than doubles your population (that increases your base income by about 40% once it fills up). Additional farms will have a rapidly diminishing return on your income, which makes it only worthwhile on very large planets. Keep in mind that a smaller increase in your base income can make a big difference because it increases the effect that all of your stock markets have. For example...

Suppose that a single farm would increase your base income by 20%, so instead of making 100 you'd make 120. You could choose between a 20% base income increase or a +30% tax bonus from a stock market. If you have no other stock markets, building a stock market would be the best course. However, if you've already got seven stock markets (random number, not necessarily the efficiency break-even point in-game) then you'll get more money from building the farm. Eight stock markets would give you a total of 340, while seven stock markets and a farm would give you 372.

Because the base tax level is determined by the square root of your population, your first farm is going to be much more effective than your second. If you're only going to be able to build one building, make it a farm unless you need the extra cash immediately for some reason. Stock markets work right away, while farms only help after the population has grown to take advantage of the higher population cap. Unless your planet quality is insanely high (I'm talking 30+ here) I'd never build more than three farms. In almost all cases, two will be more than sufficient, while every planet of even mediocre quality (7+ maybe) should probably have a single farm.