So I didn't realize that there was a FAQ/Walkthrough for this game before I started collecting values for XP needed for each promotion level. That'll teach me.
Anyhow, I was able to verify the values up to Sergeant First Class so everything looks to be correct. Because of this I produced a graph in MS-Excel to show what it looks like.
It was obviously a exponential scale of some sort, so I ended up creating a regression line of "best fit" to get an equation which models it a bit. The blue line is the line of best fit and the points are the actual XP levels. I just made the assumption that each rank corresponded to a single x-value starting at one. The equation is in the form y=A*B^x where A=10.12484807 and B=1.772377578. The "correlation coefficient r", is 0.99819747 which strongly correlates to the actual data points. Granted, this doesn't mean much as the actual equation is probably much simpler.
What was interesting was the value of 171 xp for the Sergeant rank. Everything else was either a value of 5 or 10 so I have no idea why this wasn't rounded down to 170 to fit the scope, but meh.
The FAQ/Walkthrough by Shanrak claims there is some variance in XP necessary to get promoted. Anybody see variance in XP? Because I sure didn't see that.
My X-COM Patch Kit For UFO Defense | Emergency XCOM Meeting spoof on YouTube
Zombie: Empirical data's your only man, when formulating a research plan.
A soldier's death is never in vain if it makes the formula more plain.
A few dozen make a better case for refining that third decimal place.
They call me Zombie because I don't sleep, as I slowly struggle to climb this heap,
of corpses, data points, and trials, but from the top - I'll see for miles!