How not to over-cook my fishy?

Using A1's Thai Sweet and Spicy sauce for steaming a mahi-mahi fillet. Mahi-mahi is one of our fav fish just because it taste like stingray. I still can't grasp the timing to steam my fish properly, I tend to over-cook it, also because the thickness is not uniform etc etc. Just eat lar~

Happy Memorial Day to all~