You need 2 variables, lets call one "%default xpL" and one "%default xpP"
In a loop place 2 If Variables and check if the variable '%default xpL' is larger than the Game Value 'Experience Level', if it is, then the player has picked xp and if its less then the player used xp and do the same with the '%default xpP' and the Game Value 'Experience Progress'. And after the If's, set the '%default xpL' to the Game Value 'Experience Level' and the '%default xpP' to the Game Value 'Experience Progress'