http://en.wikipedia.org/wiki/Fibonacci_numberFibonacci sequence is like
1 1 2 3 5 8 13 21
This maps onto the experience needed to gain a level
I.e.
Level, Fibonacci, Exp Needed To Gain Level
1 1 1000
2 1 1000
3 2 2000
4 3 3000
Where the exp needed to gain each level is 1000 * the Fibonacci number
This leads to exp per level ranges of
Level, Exp Range, Fibonacci, Exp needed to reach next level
0 0-999 1 1000
1 1000-1999 1 1000
2 2000-3999 2 2000
3 4000-6999 3 3000
4 7000-11999 5 5000
Etc all calculated from the Fibonacci sequence
You should really place all these values into a spreadsheet. That way you can see what experience is needed for what level. Then you can calculate the required (tasks) the player has to accomplish to get that experience versus the skills granted by the level. Assuming that you use that sort of levelling system.
For instance say there is an early boss that requires a level 3 character to beat. Before the player beats that boss they need 4000exp gained. If exp is gained via quests then their needs to be at least 4000exp in quest rewards before that boss. If the exp is gained via tasks like killing monsters then you need to calculate how many monsters the player kills to get 4000exp. Mixed systems give different results again.
If these values are calculated in the GDD then they may not need to be altered as the game is written. Of course getting the right balance is hard.