If I didnt have a computer I would find a simular round that shoots a simular weight bullet and start with the lowest load minus 10-20% and work up there. keeping in mind that a light load can be dangerous as well. I have been checking into some of the load from a disk programs were you plug in all the info and it gives you a recomended load if I get any more serious about reloading it may be the next thing I get.. in the case of the 9mmx54R wild cat there are already loads for 200 grain cast lead bulets for the 7.62x54R I would start there and work up untill primers start to flaten out and back off. my book shows a load of 20 grains of IMR4227 will push a cast 200 grain bullet at 1537 FPS this is a very light load and would likely make a good starting point that case will hold at least 52 grains of powder. I DONT KNOW ANY THING AND DO NOT TAKE MY LOADS AS FACT!!! NO ONE SHOULD USE ANY OF MY LOADS EVER!!!. If I am giving load info it is usualy out of a book and I indicate were I got it from that I will be happy to do.
Hc's loadis going to lose a fair amount of capacity with the 250 graion bullet in a standard length mag. grains of water capacity for 7.62x39 is33.98gr the 44 rem mag is 33.94 nearly identical I suspect that the loads would be pretty close. the loads for the 44 long colt might be usable as well depending on the actuall case capacity. there is also a .375 shannon that is very simular to what HC is building it has a case capacity of 32.76 it shoot a 200 grain bullet by using 22GR of 2400 powder. Im gussing that a 9x39 would be close to that (GUESSING!!)