We'll start with this post:
"After 2nd level, the amount of experience you need to gain a level goes up by 250 points
(i.e., you need +1,000 to reach level 2, +1,250 to reach level 3, etc.). After four levels
of this, the 'additional' amount increases to 500. About the point you'd expect this to continue, it gets extremely erratic on levels 11 (+1,500) and 12 (+500), then reverts back to +1,000 at 13th and 14th. Levels 15-18 add +2,000 each, and 19-20 start at +4,000 -- this implies the amount added doubles every four levels.
Except that level 21 adds +8,000, and 22 adds +3,000. After that little bit of weirdness, it does +10,000 for four levels, and +25,000 for the last four."
Here's a rough chart of the results; the first column is from the RAW, the second removes the 'jagged' parts, and the third assumes a simple doubling every time. The numbers on the latter two look funny, but the progression itself is easier to predict.
RAW Smooth Doubler
1 0 0 0
2 1,000 1,000 1,000
3 2,250 2,250 2,250
4 3,750 3,750 3,750
5 5,500 5,500 5,500
6 7,500 7,500 7,500
7 10,000 10,000 10,000
8 13,000 13,000 13,000
9 16,500 16,500 16,500
10 20,500 20,500 20,500
11 26,000 25,500 25,500 <- 500 XP difference
12 32,000 31,500 31,500
13 39,000 38,500 38,500
14 47,000 46,500 46,500
15 57,000 56,500 56,500
16 69,000 68,500 68,500
17 83,000 82,500 82,500
18 99,000 98,500 98,500
19 119,000 118,500 118,500
20 143,000 142,500 142,500
21 175,000 170,500 170,500 <- 4,500 XP difference
22 210,000 202,500 202,500 <- 7,500 XP difference
23 255,000 244,500 242,500 <- 10,500/12,500
24 310,000 296,500 290,500 <- 13,500/19,500
25 375,000 358,500 346,500 <- 16,500/28,500
26 450,000 430,500 410,500 <- 19,500/39,500
27 550,000 527,500 490,500 <- 22,500/59,500
28 675,000 649,500 586,500 <- 25,500/88,500
29 825,000 796,500 698,500 <- 28,500/126,500
30 1,000,000 968,500 826,500 <- 31,500/176,500
There are no clean mathematical formulas to approximate this, according to the curve-fitting service ZunZun.com. While my prior equation did serve to ... output the rates of increase, a lookup table or your own XP chart should serve better.
Curiously, this does highlight some interesting questions about the relative time per level, suggesting that a custom XP chart may provide a more consistent levelling experience for your players.
Edit.
Here's a quick and dirty reverse lookup that worked in excel 2008.
=VLOOKUP(A1,{0,1;1000,2;2250,3;3750,4;5500,5;6500,6;10000,7;13000,8;16500,9;20500,10;26000,11;32000,12;39000,13;47000,14;57000,15;69000,16;83000,17;99000,18;119000,19;143000,20;175000,21;210000,22;225000,23;310000,24;375000,25;450000,26;550000,27;675000,28;825000,29;1000000,30},2)