Heat lost in evaporation at constant temperature
Not a complete answer, but this should help clarify things for you:

You wrote "since the mass is decreasing during evaporation, both mass and heat would change".  But note that the change in mass per se does not affect the needed heat flow.  For instance, consider pure water.  Here, the change in mass would not affect the rate at which heat is lost due to evaporation (assuming other conditions, like surface area, temperature, the partial pressure of water vapor in the air above the water, wind speed, etc., don't change), and thus would not affect the heat flow needed to maintain the temperature.  This is because you're not trying to change the temperature of the water (in which case its mass would be relevant), you're just trying to maintain an energy balance of heat flow in and heat flow out.

Hence, regardless of the volume of water, for a specific evaporation rate, the heat flow needed to maintain constant temperature would be the same, since the latter is determined only by the rate at which heat is lost due to evaporation.  

However, since you are working with salt water solutions, their concentration would increase with time, which would decrease the evaporation rate and thus decrease the heat flow needed to maintain the temperature.

So you have two choices:

(1) Just calculate the heat flow as a function of concentration (i.e., for each solution, determine the initial heat flow needed).  If the volume of water is large relative to the surface area, treating the concentration as a constant could be a good approximation for short periods of time.

(2) Use the evaporation rate as a function of concentration to determine the concentration as a function of time. I.e., derive a time-dependent expression. This is trickier.

I'm not sure from your question which you want to do, since your concern seemed to be the change in the total mass of the water, not the concentration. Note also that the equation you found in the linked article doesn't include a variable for concentration.