Experience is pretty self-explanatory. You acquire experince completing almost every action in-game. Eventually gaining enough to level-up.
After doing some in-game research you will notice that experience per level increases sequentially (it takes 16cxp to get to lvl 8, 17cxp to lvl 9, 18cxp to lvl 10 and so on). The only exception is the first 6cxp to get to level 2 then it jumps to 11cxp to get to level 3.
Therefore there is a simple formula
current card experience =((lvl+9)*(lvl+10)/2)-49-cxp)
experience needed to max out card =(((stars*10+8)*(stars*10+9)-(lvl+9)*(lvl+10))/2)+cxp
lvl = Card's current level. cxp = CXP needed to next level. stars = Card's stars.
Do note that this these formulas work on level 1 as well.
If you (like me) are too lazy to type this in a calculator every time go to this link and type in your information. http://tinyurl.com/ptlexp