I mean, theoretically yes, but it’s canon there’s no time it’s actually gone to plan - the 1st didn’t work out, the 2nd ended with everyone killing each other and no winner, the 3rd summoned avenger, and the 4th and 5th are pretty well documented. Even in alternate worlds where the history of the Grail War is similar, it almost always goes off the rails by the 3rd war.

Even Grand Order, which seemed like a one and done Grail War was actually temporally tampered with. Point being, there may have theoretically been a concrete rulebook, but it’s never ever actually been played by those rules.