since its 50 caps, you'd need 10g of dnp, and since a 0 cap its roughly 550mg of space, you'd add the effective amount of 350mg of (since we're talking dnp) potassium per cap so 17.5g of that. Helps if you know the density of the powders, then you can calculate it even more precisely. For instance vitamin C powder is way denser than most herbals, so in this instance if using that i'd expect to be around 450mg of vit c to 200mg dnp per cap. So you take the 17.5g of potassium, the 10g of dnp, put it in oh for instance an empty pill bottle that is dry and shake it heavily. so long as neither powder is clumped to begin with, you get a pretty even distribution. 100% perfect? no, but as close a tolerance as is used in production of supplements anyhow. I never run batches of just 50, in this case i'd probably go down to 100mg dnp per cap (if you just had 10mg) and make it 450mg of potassium or whatever per cap for 100 caps. This way you can run the first batch of 50 and weigh a few (subtracting empty capsule weight of course) to see how much total of mix you got per cap, and get an idea of variation. You get more variation with materials that flow poorly, clump, or pack easily.
I've tended to find that I like using the 00s, they are around 750mg capacity with whey or creatine mono, and I always have those around to use as filler