I have done an research to find out the formula for the Population info shown in the Demo screen, finally i made it. And the formula should be:
P = Max(1000, (N(N-1)/2)*(N+4)*1000)
N is the Population you have, P is the pop shown on the Demographic screen
e.g. If your game has just been started so your pop is only one, then your pop points in demo screen is, Max(1000, (1(1-1)/2)*(1+4)*1000), take the larger figure so 1000 is taken.
e.g. If you are in an early game and you have only your capital now and it has 3 Pop, then the Pop shown in the demo screen should be Max(1000, (3(3-1)/2)*(3+4)*1000) = 3*7*1000 = 21*1000= 21000
But it's a bit strange when you have a pop of 7, it doesn't show exactly 231000 but 232000 instead, i'm not sure where is the extra 1000 points come from, at first i thought it might be because of i made an worker so i guessed 1 worker = 1000 points, but then i made another worker but the value didnt change, then i made 3rd and 4th, still the same.
What makes it stranger is the problem
disappears when you reached the pop of 10, it shows EXACTLY 630000 now, then the problem is, why the pop points have extra 1000 points when you have a pop of 8 and 9, but soon disappears as soon as you reached pop 10? Maybe the answer is the game have a similiar but not exactly the same formula as mine. Maybe there are some other factors which also affect the pop points?
Anyway it's great to have found a formula by myself.
I had forgotten that! 
