That's what playtesting is for. :]
Seriously tho, the only real way to get a good feel for the curve is just to have as many people as you can play through the game as normal and give feedback on points in the gameplay loop where the levels felt a bit inelegant. As for just planning it, again, you can always theory craft what level you'd be at at any given point in the game, but playtesting best identifies this. Some other factors to keep in mind are things like the Exp. Share All, distribution of EXP Candies, etc.