Developing any load for a custom case, or even a new chambering for you, is mostly common sense.
As was said, the easiest thing to do is find a case that has a similar capacity, and start with that load data. A Chronograph is essential.
You already have a good idea what velocities you are looking for, so approach this figure, keeping an eye on what the rifle is telling you. Bolt lift and the outside radius of the primer are still good indications as to what is happening.
As far as powder goes, the general rule is the powder that gives as close to 100 percent density at the velocity you are looking far gives the best results. As a good example, in the 6PPC, 133 continually gives great results. The loads most of us shoot have to be loaded with a long drop tube at a slow trickle. That is 100 percent density. That’s what works.
In the 30BR, shooting 112 grn bullets, 4198 continues to be the go to powder. My favorite load is 34.8 grns behind my 112. The bullet is touching the powder when seated.”
I am also working with my regular 6BR, .060 throat. Bart’s 80’s in a 1-12 twist barrel loves Accurate 2495, 33,5 grns. This is getting close to a 100 percent density load. With my own 103 grn in a 8 twist, I was surprised when I tried N150. It should be too slow, but 30.1 grns gives a flat 2900 FPS with great accuracy. It is not a 100 percent density load, which leads me to believe that it is a little faster than the Burn Rate Charts indicate.
Keep in mind. The laws of physics don’t get repealed with every new idea. If you develop a case that has the same capacity as an already existing case, you are not miraculously going to find another 200 FPS with all other perimeters being equal. You can develop something with a little more efficiency, but velocity is still achieved by a certain amount of pressure behind a certain bullet out of a given length of barrel.