The large can of 6-in-1s that Steve has weighs (6 x 16) + 9 = 105 ounces. If you want to scale down Steve's seasonings mix for one 28-ounce can of tomatoes, whether they are 6-in-1s or the Classico crushed tomatoes, you would use 28/105 = 0.267 times of the amounts of the individual ingredients of Steve's seasonings mix. Since Steve gave most of the ingredients of the seasonings mix by volume rather than weight, you are likely to end up with some odd volume numbers for the scaled down amounts. For example, 0.267 x 1/2 teaspoon = 0.1335 teaspoon. How do you measure that?
I think the better approach is to convert all of Steve's ingredients specified by volumes to weights and then calculate the percents of all of the ingredients relative to the weight of the tomatoes in the large can of 6-in-1s (much like baker's percents relative to flour). Of course, to do this, you would need to have a digital scale that can measure small amounts of lightweight ingredients like herbs. You would also want to know if Steve measured out the volume measurements of his ingredients as heaping measuring spoons or level measuring spoons. But, once you get the weights of all of the ingredients in the seasonings mix, you can calculate the total weight and, for one 28-ounce can of tomatoes, you would uses 0.267 times that weight. You might be able to use a regular scale to measure out that amount. Of course, you would have a fair amount of the seasonings mix left over. But, that is not necessarily bad. The seasonings mix should be usable for several experiments, including those used for the Classico crushed tomatoes for side-by-side tests. The advantage of using the baker's percent approach is that you can use those numbers relative to any amount of tomatoes, from a thimbleful to a trailer car amount.
BTW, you might also ask Steve what form the oregano, basil and thyme take in his mix. Are they leaf or ground? And does he measure out the herbs right out of their respective bottles or does he pulverize them in any way, as between the fingers? Some herbs are quite potent and can throw off a sauce if used in a ground form rather than a leaf form.