Endurance formula help needed...

I've been working on my epic RPG for a good while now and I wanted to add something different that no one else has done. The idea is that no matter how hard you've worked to create a new character with stats that you like, it comes out crappy and you don't feel like going through the process again to make a new character. What do you do? Go to the Gym! The gym will be a separate level where you do mini games to augment your stats. For each point of endurance, you move the slider bar one tick. My issue is that endurance in my game is similar to Skyrim. For each attack, it depleats your endurance until you have to either try to get away or die trying. Endurance is a calculated stat, but I'm not sure the best way to do it. I know it has something to do with stamina as a base, but constitution, and strength play a factor, but I'm not sure exactly how. If anyone is willing to help, I'd gladly put you in the credits once the game is complete. Thanks in advance all!