Who is your professional help... can they answer these questions?
Temperature: I think this would be a tradeoff between maintaining a sufficient temperature but minimizing vaporization. You will have more vaporization at a higher temperature, but you will probably also see better mixing or the copper and zinc. Theoretically, the minimum temperature necessary to achieve a molten state should be no higher than the standard melting point of copper (freezing point depression caused by adding zinc).
I don't know how long to wait. I don't see any benefit in waiting past the "full melt" point.
The % lost due to vaporization will depend on time, pressure, temperature, composition, etc. You could assume that at the interface of your liquid metals and atmosphere, the concentration of zinc vapor is equal to its vapor pressure at the temperature. Then you need to know how much the atmosphere is moving to determine the mass transport of zinc vapor.