The "best" ratio of sauce depends on:
- The type of vegetables;
- The type of sauce;
- The quantity of solid food;
- Time spent cooking;
- Personal preference.
I've made some stir fries with no sauce at all (technically a stir-fry only needs oil) and some with extra sauce, particularly if they're intended to be eaten with bland white rice. I know you say you can't trust their judgment, but to some degree, you're really going to have to, because there are too many variables to pin it down to a golden ratio.
At the end of the day you're trying to get an even coat on the vegetables (or meat, or whatever) - the thickness of that is again dependent on personal preference. But even if you make that decision for them, the main ratio affecting the end result isn't based on the weight of the food, or even its volume - it's based on surface area, which is nigh on impossible to measure and guaranteed to be inconsistent between specific preparations.
Stir-fry sauce is extremely cheap to make. It's pennies per cup. If you make too much, you can always cook it down, so if you have to pick a number, it's better to err on the side of caution and make too much as opposed to too little. You can always supplement the real sauce with soy sauce, teriyaki, etc., but that's not the same as a proper stir-fry sauce thickened with tapioca and flavoured with sesame oil and so on.
So pick a number that seems a little high, and instruct the cooks to let it reduce if it seems watery. They'll be using high heat, so it won't take long to reduce, and the net effect is positive anyway (a stickier, more flavourful sauce).
Personally, I always find 1 generous cup of sauce to be more than enough for a standard-size (14") wok filled with meat and vegetables, assuming it's the kind of stir-fry that you want to be reasonably "saucy", and also assuming it is thickened properly; if you dump a full cup of pure soy sauce in there, you'll just end up with vegetable soup. I literally use a coffee mug and never measure any of the ingredients, and the difference between one preparation and the next isn't particularly noticeable.
So start with that as a guideline and test the recipe yourself, if you can. Unless you have extremely precise control over the quantity and quality of the raw ingredients in addition to a precise and well-tested sauce recipe, you won't be able to come up with a reliable ratio. It's not a question of trust, just practicality; Asian cooking is (at least in my experience) very loosey-goosey and doesn't lend itself well to precise recipes - or inexperienced cooks.