Calculating the specific gravity of a blend


So I am trying to make a starter for brewing Thursday but the yeast smack pack I got was about 90 days old so the viability from the brewers friend calculator is only 37% or 37 billion cells. Using the Brewers friend calculator I found that I could get a proper pitch for the 10 gallon batch if I did 2 steps the first one a 1L starter on the stir plate with an OG of 1.030 (which I have already started) and then a 2L starter at 1.040. The problem is I don't have enough time to crash cool between starters before the brew day so I was thinking tomorrow I could just add another 1L of a stronger wort to get to approximately 1.040. I was thinking the current 1L starter is just pale malt and should end up somewhere around 1.010 so if I add another liter of 1.070 wort I will end up at 1.040. Here is the math 1.010+1.070=2.080/2=1.040, since they are both the same volume that does not need to be calculated. Is this correct or is the formula for calculating the specific gravity of a blend more complicated than that? I know this isn't going to be exact but I'm just trying to get somewhere close to a proper pitch rate. Thanks.
