I only have 00 caps and alot of them, so for the herbs you say 750mg per 00 cap, would filling the larger end only equate to 500mg do you reckon.
750mg is an average. The problem is that different herbs have different densities. And the milligrams are weight, not volume. So a capsule of let's say kelp will be a much higher weight than the same size capsule of mullein, which has a lower density.
You don't have to be exact on the 500mg. But if you want to be the best way to determine this would be to weigh out a specific amount of the herb, such as 1g then see how many capsules it fills on average, then you can divide the number by two to find out the average capsule weight. Of fill a bunch of capsules and see how many it takes to reach 1 gram. One gram is usually the minimum most small scales will register.