Well, the long-practiced method is to look at powders that are about the same burn rate in that cartridge and begin with the start loads. Comparing results with those of the known powder(s) tells you if the unknown powder is a bit faster or slower in that cartridge. And you adjust from there. In the 7-30 Waters, there are published loads for powders like W748 and H335 (it's important to compare ball to ball and stick to stick powders only), so it shouldn't be too hard.
