How much flux i have to use?

[user 72133]

I have smelting different types of brushes (I don't know if it's said that way) I mean filings, polishing rubbish, jewelry crap... something that have precious metals and other things without value. what I normally do is burn everything and smelting the ashes. I am using for every 2kg of ashes, 1kg of borax and 1kg of sodium carbonate. I use that amount because I heard about it but I would like to know if you are doing this type of recovery,
-how much flux do you use?
-what kind of flux?
-It is different for each kind of residue?
-what i have to look before smelting to know what or how much flux i have to use?
-Do you use any pretreatment before smelting it?
Thanks