The variables in firing any kiln are part of it- especially a flame firing as opposed to an electric kiln or one where the ware is fired in saggars (clay boxes)
The thing is to work with the variables.
The flame path within the kiln- you can pack the pots to work towards what you want .
You can also place little dishes with a little salt in them and this will volatalise and give you a salt glazing even without adding any salt through the spy holes. This is good where you think that a particular part of the kiln might be being missed.
The weather is the biggest variable with the air pressure,wind and humidity affecting firing whether you are salt firing or not.
You can work towards the result you desire and the top potters in this field can get a 95% success rate or more. (industrial production is a different matter and a different product)
However, we can't make a living with this, so we can get together and share and learn a few times a year. What we learn can be reflected in the rest of our work. Sharing the enthusiasm can fire the inspiration in a completely different field of work.
We are still just beginners and thank people like Rosemary Cochrane and Wally Keeler for their knowledge that helps us do this