The Exp Formula
I deduced this formula with blood, sweat, and tears... O.k. just a lot of sacrificed nymphs and silver. It's a simple piecewise function.
x = level-1
This shows how much experience you have if you don't have any left over exp.
Estimating Max Stats
First of, if you look any card's starting stats for heroic, legendary, or epic you will notice that it is a direct ratio from the base card. They are as follows:
3 stars heroic : basic = 3:1 legendary : basic = 6:1 epic : basic = 12:1 4 stars legendary : basic = 3:1 epic : basic = 6:1 5 stars epic : basic = 4:1
If you know the base max stats (which a lot of muses gave their lives for), then you can calculate a good estimate of what the others are going to be and vice-versa. The margin of error I've found to be no more than a percent. I have noticed that 3 star cards tend to be lower where 4/5 star cards tend to be higher than the actual amount, but this could be just the cards that I have looked at.