I am having a pointless argument with someone about a rural water system. Because of the current dry spell, the rainwater tank is low and it needs to be supplemented with spring water. The problem with spring water (and the reason for the rain water tank) is that the spring water is very hard, with a high calcium content. This buggers things like the wetback and hot water heater. 

The argument: The other party insists that the spring water should be added while there is still rain water in the tank, as this will dilute the spring water, reducing the calcium content and bad effect thereof. I say it makes no difference when the spring water is added as long as the total volume of spring water remains the same. If it is initially mixed with rain water, the calcium content will be less initially but only until all the water is used up. If spring water is only added after all the rain water is gone, the calcium content will be higher but total exposure time to the calcium will be less since it wasn't added until after the rain water was all used up. The assumption in both cases is that spring water would continue to be used until it rains.

As far as I can see, both cases are just different ways of getting to the same point. In the first case the spring water is blended with the remaining rain water. In the second case, it is added after all the rain water has been used. In both cases, the total amount of calcium put into the system would be the same. The only exception would be if it happens to rain just after the tank runs dry but just before the spring water is added. 

So which is it? Does it make any difference calcification-wise whether the hard water is blended or added afterwards?