The right "ratio," such as it is depends on what kind of rice you're using and how much rice you're cooking. For most rice, you may use the "1 knuckle trick," which is basically 1:1 + some extra water.
The way I learned was to cook, dump the rice when it was done onto a sheet pan, and fan, fan, fan, the steam off so it couldn't re-condense on the rice. That was before electric rice cookers were common though. Doesn't exactly seem practical for a group in excess of 150 either, does it?
You MUST use an appropriate type of rice, You CANNOT use a basmati or ordinary long grain type. You SHOULD NOT use aged or old rice. Regardless of grain shape, you need to use something more glutinous. Other than saying I like to use CalRose which is an American grown japonica, I don't know what's available to you.
You should season the rice with vinegar, sugar and salt while it's still warm.
Under the most ideal circumstances, holding the types of rice used for "sushi" is an issue. But holding seasoned rice in quantity, for more than a couple of hours without seriously compromising quality... no flipping idea. You've got to keep it moist, reheating will turn it into sticky mush. There must be tricks, but I don't know them.
Your best bet might be to rent a bunch of rice cookers, schlepp them along, cook and season onsite, and hold in the cookers with a damp towel under the lid.
Good luck,
BDL