RL-15 is a good powder for your .404 Jeffery, but I too prefer VARGET for all bullet weights I use: 340-, 380-, and 400-grainers
I have a 24" McGowen-barreled Winchester M70 Stainless Classic (0.425" groove and 1:10" twist),
same barrel basics as used in the CZ 550 Magnum factory 404J rifles with 25" length, I have one of those too.
If you have a tighter-grooved barrel you may get higher velocities/pressures than I did with my modest loads in the McGowen barrels.
You might use a grain or two less powder to do the same.
RL-15 and Varget are close to same performance.
I got better accuracy and uniformity with Varget, and it might be more temperature stable.
Also, I have experienced more lot-to-lot variation with RL-15 than with Varget.
400-gr Woodleigh RNSP, Norma Brass, F215, bullet crimped on Woodleigh cannelure, 82 degrees F, 24" McGowen barrel.
Starting loads, 5-yard chronograph, no correction to MV:
RL-15
72 gr >>> 2120 fps
75 gr >>> 2227 fps
77 gr >>> 2290 fps
79 gr >>> 2360 fps
81 gr >>> 2428 fps
VARGET
72 gr >>> 2125 fps
75 gr >>> 2233 fps
77 gr >>> 2275 fps
79 gr >>> 2336 fps
81 gr >>> 2406 fps
H4350
85 gr >>> 2276 fps
86 gr >>> 2304 fps
87 gr >>> 2329 fps
380-gr NorthFork FP copper solid, 86 degrees F on a different day, seated to and crimped at first driving band,
same rifle and load specs otherwise:
H4831SC
88 gr >>> 2234 fps
90 gr >>> 2285 fps
380-gr North Fork Soft Point, 83 degrees F on another day, COL 3.590", same 24" rifle and other load specifics as above:
Varget
83.0 gr >>> 2528 fps FAVORITE BISON KILLER LOAD, 1-MOA for 3 shots at 100 yards.
340-gr North Fork Soft Point, 54 degrees F on another day, COL 3.554", same 24" rifle and other specifics as above:
Varget
87.6 gr >>> 2706 fps FAVORITE SHEEP HUNTING LOAD, 0.5-MOA for 3 shots at 100 yards, 0.6-MOA for 3 shots at 300 yards.
Range-tested trajectory:
Zeroed 3" high at 100 yards
Strikes 0.5" high at 200 yards
Strikes 7.75" low at 300 yards
The 400-gr Kynoch FMJ at ~2400 fps with 81 grains Varget
and the 380-gr North Fork SP at ~2525 fps with 83 grains Varget
and the 340-gr North Fork SP at ~2700 fps 87.6 grains Varget: All shoot 3" high at 100 yards with same scope setting in my rifle.