I hear so many people say: "If I make x amount of sales at x price, I'll make x monthly! That's all I have to do!"
Or, just make ten sites making $10 a day, and that's a hundred bucks a day!
Or, if my product converts at x% and I get xxxx visitors, I'll make $xxx!
PUT AWAY THE CALCULATOR, STUFF THE THEORY ASIDE, AND GO DO SOMETHING!
If I told you how many times I whipped out the calculator when I first started, and said "that's all I have to do", it would be in the hundreds.
If I told you how many times it actually worked out like I planned it to, it would be in the ball park of 0-1.
Things rarely go by the numbers, so your best bet is just hide the calculator until tax time and just get out there and hope for the best, and prepare for the worst.
But maybe thats just me.