You cannot calculate it, you can only measure it.
Scoville is a subjective scale. Wikipedia tells you how it is measured - by testing with human panelists, using a certain protocol.
As all subjective scales, Scoville is ordinal. Even though it is expressed in numbers, these numbers are best understood as ranks - you cannot assert that "1000 SHU is twice as hot as 500 SHU", for example, or that "the hotness difference between 500 SHU and 1000 SHU is the same as between 1000 SHU and 1500 SHU".
If you take statistics, you will find that many methods are treating ordinal scales as if they were interval, and go on calculating averages and whatnot. This tends to work well enough for some applications. But in your case, you are dealing with human sensory perception. I am not 100% sure for the specific case of hotness, but typical human responses to a stimulus are logarithmic, not linear. So if you try to simplify your life and make a linearity assumption for Scoville, your estimates are likely to be so far off as to be unusable.
If you really need an exact Scoville number for your sauce, it is not math you need, but the resources to conduct proper testing in several iterations. I fail to see the relevance of such goals for the home cook though. You eat sauce, not numbers.