I could post how I got this, but all my math was written on paper. I figured I'd share the info.

Using max BPT Formula, using exclusively explore and max teching I wanted to see at what turn does each group break even:

These are my findings:

196 turns @ 2k land (1840 labs) (aprx 600 tpt)
326 turns @ 4k land (3600 labs) (aprx 1100 tpt)
568 turns @ 8k land (7600 labs) (aprx 2522 tpt)

@ turn 482 the person with 4k land breaks even with the person with 2k land
@ turn 755 the person with 8k land breaks even with the person with 4k land

Enjoy.

What's the current construction per building formula?

My math is nothing more than trial and error. With enough error I figured out decent math :)

How many CS are you assuming each of these three countries have at these turn counts (same country, different times)?

2k land @ sqr (2000 / 4) = 23 bpt
4k land @ sqr (4000 / 4) = 32 bpt
8k land @ sqr (8000 / 4) = 45 bpt

To get CS you use (CS/4)+5 = BPT (sub in BPT) & (solve for CS)

example:
(CS / 4) + 5 = 23
(CS / 4) = 18
CS = 72