Custom water mains temperature workaround?

I am trying to model a solar water heating system that can boost the temperature of water in a tank from a heat recovery system, as well as pre-heat water from mains. It appears that there is no way to set a custom hourly mains temperature. However, I am wondering about a possible workaround.

Suppose I have a tank of water containing sitting at 500 gallons at 140 degF, which I must raise to 170 degF. (dT=30)
Suppose that I need an additional 500 gallons of water raised from mains temperature (50 degF) to 170 degF. (dT=120)

It is straightforward in SAM to simulate raising 500 gallons of mains water by 120 degF.

To simulate that I am ALSO heating the 500 gallons of 140 degF water in the tank, will I get a reasonably close answer if I simply regard that volume as though it were a much smaller quantity being raised by 120 degF, rather than the full volume being raised by 30 degF?
i.e.
[(170-140)/(170-50)]*500 = ...
(30/120)*500 = ...
0.25*500=125 gallons

Thanks for your help.

Thank you for your response. Since there is no way to manually input water mains temperatures, my question is really about the possible work-around.

What I'm trying to get at is this:
Since I cannot change the water mains temperatures, will SAM give me a *reasonable approximation* if I simulate my process by simply reducing the volume of the water draw by the appropriate amount?

So, if the volume of water X that I actually need to heat is already half-way to my target temperature set by the auxiliary set temperature, I would input that I draw a volume of water X/2.

Would that work?