No matter the mixing method, the fiber to castable ratio is:
1 BAG OF FIBERS -per- 1 BAG OF CASTABLE REFRACTORY.
The problem with mixing Castable Refractory in a cement mixer is the refractory starts to adhere to the mixing paddles… so we recommend mixing no more than 2 bags of castable refractory (in a mixing bucket / pail) at one time.
Additionally, it’s considerably easier to gauge / judge the amount of water you add to the refractory as you mix the refractory w/ COLD water. And refractory needs VERY LITTLE water… just enough to pass the Ball in Hand method.
Here’s a great article about mixing Castable Refractory w/ Fibers & water…