When starting new load development i use every reloading book i own. I read the loads and write down the max/min and coal from each book. I then find the mid range load from all of this and start there. I have found my most accurate rounds to be near the max load. I then subtract Max from my Med load and then divide that to get 5 total loads. I then load 5 rounds each and shoot each at a different target. The one that groups the best (Say load 3) ill take load 2-4 and make 5 loads between 2-4 (Not including the 2,4). I then shoot and get the best group. I continue until i get the best group. From there I play around with seating depth. Once i get a good seating depth ill move around the charge range a little bit more. For me its long amounts of trial and error, but i get lots of shooting practice in doing this.
Always shoot through a chronograph, record each velocity, they should all be close, you will also find that your lowest ES is probably your most accurate load (In my rifles it is).
Ill be starting my load development for a 338-378 wby here soon have some 300 bergers but thinking about moving down to a 225 or so. Ill be using H870 (Found 2 cans sitting at the floor of the home FFL who transfered the rifle, and was able to get 2# for 20
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