1 part soy sauce (low sodium if you have it)
2 parts Dark soy sauce*
Salt & Pepper
For those unfamiliar, dark soy sauce is NOT regular soy sauce but it's own thing. Soy sauce is either light or dark. Regular soy sauce is light (usukuchi). Dark (koikuchi) is aged with molasses and has a little bit of cornstarch.
Where light is often added to dishes after they've been cooked, dark is used almost exclusively for cooking and marinades. It is also used in "red cooking."
Blackstone fried rice is one of our favorites. You are spot on with using leftover rice. The fridge dehydrating it makes a HUGE difference. I also tend to use some oyster sauce to help with the umami and cut down on the sodium a bit. It's such a versatile dish and the griddle is a great way to cook it.