You can also use the current value. Now that I looked back, I don't think I've mentioned it anywhere lol.
Anyways, you should be able to do something like:
Lets say we have 100 attack, then we add the param plus:
atk: current * 1.25
That will add an additional ( 100 * 1.25 ) for a total of 125. So our new Atk will be the sum of 225.