You can estimate the SHU by splitting the mass of the product and mixing it within a ratio.
For example, 24kg of 6mil SHU would need to be added at a 1:6 ratio if your goal is a final product with 1mil SHU.
That is for every 1kg of capsicum you would need to match it with 6kg of soybean oil. So 24kg would in theory make 144kgs of 1 million SHU final product.
I will say that it is challenging to calculate exactly without testing it. Scoville ratings are defined per unit of dry mass, but the comparison of ratings between products having different water content within the solution can mislead those numbers.
For instance, if the solution is soybean oil-based, the spiciness will be slightly dulled by the oil even if it is at 1 million SHU.