Rashid:  I don't believe you are going to achieve what you want by adjusting the fluxes, although I am not an expert on them.  I do know you can spin your wheels with numerous experiments that deal with specific ores/concentrates/fluxes and then if or when you perfect them, your ore can change slightly and you loose the value of your experiments as pertains to your goals or you start to experiment with corresponding adjustments to the fluxes to deal with ore changes which can occur from property to property or even within given properties.  I have worked with leach processes at ambient temperatures of 70 degrees F up to 100, that applied to extremely fine gold all the way to just prior to colloidal, that allow one to successfully separate the silver and other metals before ever smelting.  The principles should also be applicable to larger material, perhaps allowing for longer digestion periods.      

It is conceivable that you can accomplish the same, unless your gold concentrates consist of really large particles and contain gold/silver alloy, in which case you might explore running them through a ball mill first and then either cementing the silver out of the cons first, or using the following suggestions.  If your cons are -20 mesh or so, then you may want to explore adding an additional process post concentrate, but pre-treatment prior to the smelting process, to your work.  Rather than try to increase Gold Purity during smelting, why not set up a separate pre-treatment for your concentrates?  For example, it you run them first through a nitric bath, thus removing the silver and perhaps other contaminant or base metals and then run the remaining material, post bath, through your smelting process -- by then it may be pure enough (70% Gold minimum) to use a flux like the Chapman formula. 

Also, you could then drop the silver separately from the nitric using plain salt, thus  then having both the Gold and Silver in much more salable purities.  If there are PGM's in there as well, you can also run this through an ammonium chloride process to remove most of them prior to smelting.  Bear in mind that the sequence of which to drop first is important and you may need to consult an expert as to which to do first.  The PGM process results in a black powder, which can be sold to a refinery, or processed/separated/refined in your own lab if you have a more sophisticated setup and want to dedicate the time and resources to them.  I personally hate working with nitric because it is very hard to kill, but it does serve a purpose here.  It will require specific knowledge in order to wash the nitric out of solution, prior to drying and smelting.  I love silver as a metal or investment, but it can be a royal pain, but very necessary one to process.    

You can further refine any or all of these precious metals by running a follow-up bath(s) to further clean them if you want -- it depends somewhat on your operation and cost/availability of the extra setup, equipment, workers, and reagents, etc., to comfortably and economically make it all work.  If gold is your main goal, you can stockpile the silver powder, PGM's, etc., and depending on your operation, you can either run a sporadic out-doors process every several months or even years, or simply sell them to another processor, larger mining concern, or refiner.  Some of this work will require knowledgeable and trained workers and advanced lab including fume hood, etc., and can also cause one to deviate from their objectives.  It is very important that any acids, nitric or other be fully removed and/or neutralized prior to drying material to ready for smelting.

Depending upon quantities of cons involved and your desires, your economics will ultimately dictate your decisions and the direction of appropriate processes and procedures.  In many cases, people simply sell the silver, platinum group, rare earths, and base metals to others who are equipped to deal with them and then use those proceeds to pay for much of the costs of the entire operation, leaving the gold as primarily a "free" or "profit" item.  Also your location may deem that your costs of environmental compliance may prove to be prohibitive if you perform some of this stuff where laws are very strict -- they are normally that way for good reasons, and will become more and more so as time goes on.  Mining itself is demanding this in order to protect the planet, while still providing essential products at a profit to the masses.

Hope this is helpful to your endeavor, but feel free to inquire if I or anyone else can provide further ideas or assistance -- good luck out there!